Fix newline character in test attachment string for Windows

Updated the base64-encoded string for attachments in the tests to reflect the correct newline character for Windows. This ensures that the test cases correctly validate the presence of attachments in the mail messages.

client_test.go

@@ -30,8 +30,6 @@ import (
 const (
 	// DefaultHost is used as default hostname for the Client
 	DefaultHost = "127.0.0.1"
-	// TestRcpt is a trash mail address to send test mails to
-	TestRcpt = "couttifaddebro-1473@yopmail.com"
 	// TestServerProto is the protocol used for the simple SMTP test server
 	TestServerProto = "tcp"
 	// TestServerAddr is the address the simple SMTP test server listens on
@@ -3528,9 +3526,9 @@ func parseJSONLog(t *testing.T, buf *bytes.Buffer) logData {
 }
 
 // testMessage configures and returns a new email message for testing, initializing it with valid sender and recipient.
-func testMessage(t *testing.T) *Msg {
+func testMessage(t *testing.T, opts ...MsgOption) *Msg {
 	t.Helper()
-	message := NewMsg()
+	message := NewMsg(opts...)
 	if message == nil {
 		t.Fatal("failed to create new message")
 	}

msg_unix_test.go

@@ -120,28 +120,27 @@ func TestMsg_WriteToFile_unixOnly(t *testing.T) {
 	})
 }
 
-/*
-func TestMsg_WriteToTempFileFailed(t *testing.T) {
-	m := NewMsg()
-	_ = m.From("Toni Tester <tester@example.com>")
-	_ = m.To("Ellenor Tester <ellinor@example.com>")
-	m.SetBodyString(TypeTextPlain, "This is a test")
+func TestMsg_WriteToTempFile_unixOnly(t *testing.T) {
+	if os.Getenv("PERFORM_UNIX_OPEN_WRITE_TESTS") != "true" {
+		t.Skipf("PERFORM_UNIX_OPEN_WRITE_TESTS variable is not set. Skipping unix open/write tests")
+	}
 
+	t.Run("WriteToTempFile fails on invalid TMPDIR", func(t *testing.T) {
+		// We store the current TMPDIR variable so we can set it back when the test is over
 		curTmpDir := os.Getenv("TMPDIR")
-	defer func() {
+		t.Cleanup(func() {
 			if err := os.Setenv("TMPDIR", curTmpDir); err != nil {
 				t.Errorf("failed to set TMPDIR environment variable: %s", err)
 			}
-	}()
+		})
 
 		if err := os.Setenv("TMPDIR", "/invalid/directory/that/does/not/exist"); err != nil {
-		t.Errorf("failed to set TMPDIR environment variable: %s", err)
+			t.Fatalf("failed to set TMPDIR environment variable: %s", err)
 		}
-	_, err := m.WriteToTempFile()
+		message := testMessage(t)
+		_, err := message.WriteToTempFile()
 		if err == nil {
-		t.Errorf("WriteToTempFile() did not fail as expected")
+			t.Errorf("expected writing to invalid TMPDIR to fail, got: %s", err)
 		}
+	})
 }
-
-
-*/

msgwriter_test.go

@@ -6,151 +6,673 @@ package mail
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
 	"mime"
+	"runtime"
 	"strings"
 	"testing"
 	"time"
 )
 
-// brokenWriter implements a broken writer for io.Writer testing
-type brokenWriter struct {
-	io.Writer
-}
-
-// Write implements the io.Writer interface but intentionally returns an error at
-// any time
-func (bw *brokenWriter) Write([]byte) (int, error) {
-	return 0, fmt.Errorf("intentionally failed")
-}
-
-// TestMsgWriter_Write tests the WriteTo() method of the msgWriter
 func TestMsgWriter_Write(t *testing.T) {
-	bw := &brokenWriter{}
-	mw := &msgWriter{writer: bw, charset: CharsetUTF8, encoder: mime.QEncoding}
-	_, err := mw.Write([]byte("test"))
-	if err == nil {
-		t.Errorf("msgWriter WriteTo() with brokenWriter should fail, but didn't")
+	t.Run("msgWriter writes to memory for all charsets", func(t *testing.T) {
+		for _, tt := range charsetTests {
+			t.Run(tt.name, func(t *testing.T) {
+				buffer := bytes.NewBuffer(nil)
+				msgwriter := &msgWriter{
+					writer:  buffer,
+					charset: tt.value,
+					encoder: mime.QEncoding,
 				}
-
-	// Also test the part when a previous error happened
-	mw.err = fmt.Errorf("broken")
-	_, err = mw.Write([]byte("test"))
-	if err == nil {
-		t.Errorf("msgWriter WriteTo() with brokenWriter should fail, but didn't")
+				_, err := msgwriter.Write([]byte("test"))
+				if err != nil {
+					t.Errorf("msgWriter failed to write: %s", err)
 				}
+			})
+		}
+	})
+	t.Run("msgWriter writes to memory for all encodings", func(t *testing.T) {
+		for _, tt := range encodingTests {
+			t.Run(tt.name, func(t *testing.T) {
+				buffer := bytes.NewBuffer(nil)
+				msgwriter := &msgWriter{
+					writer:  buffer,
+					charset: CharsetUTF8,
+					encoder: getEncoder(tt.value),
+				}
+				_, err := msgwriter.Write([]byte("test"))
+				if err != nil {
+					t.Errorf("msgWriter failed to write: %s", err)
+				}
+			})
+		}
+	})
+	t.Run("msgWriter should fail on write", func(t *testing.T) {
+		msgwriter := &msgWriter{
+			writer:  failReadWriteSeekCloser{},
+			charset: CharsetUTF8,
+			encoder: getEncoder(EncodingQP),
+		}
+		_, err := msgwriter.Write([]byte("test"))
+		if err == nil {
+			t.Fatalf("msgWriter was supposed to fail on write")
+		}
+	})
+	t.Run("msgWriter should fail on previous error", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter := &msgWriter{
+			writer:  buffer,
+			charset: CharsetUTF8,
+			encoder: getEncoder(EncodingQP),
+		}
+		_, err := msgwriter.Write([]byte("test"))
+		if err != nil {
+			t.Errorf("msgWriter failed to write: %s", err)
+		}
+		msgwriter.err = errors.New("intentionally failed")
+		_, err = msgwriter.Write([]byte("test2"))
+		if err == nil {
+			t.Fatalf("msgWriter was supposed to fail on second write")
+		}
+	})
 }
 
-// TestMsgWriter_writeMsg tests the writeMsg method of the msgWriter
 func TestMsgWriter_writeMsg(t *testing.T) {
-	m := NewMsg()
-	_ = m.From(`"Toni Tester" <test@example.com>`)
-	_ = m.To(`"Toni Receiver" <receiver@example.com>`)
-	m.Subject("This is a subject")
-	m.SetBulk()
+	msgwriter := &msgWriter{
+		charset: CharsetUTF8,
+		encoder: getEncoder(EncodingQP),
+	}
+	t.Run("msgWriter writes a simple message", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
 		now := time.Now()
-	m.SetDateWithValue(now)
-	m.SetMessageIDWithValue("message@id.com")
-	m.SetBodyString(TypeTextPlain, "This is the body")
-	m.AddAlternativeString(TypeTextHTML, "This is the alternative body")
-	buf := bytes.Buffer{}
-	mw := &msgWriter{writer: &buf, charset: CharsetUTF8, encoder: mime.QEncoding}
-	mw.writeMsg(m)
-	ms := buf.String()
-
-	var ea []string
-	if !strings.Contains(ms, `MIME-Version: 1.0`) {
-		ea = append(ea, "MIME-Version")
-	}
-	if !strings.Contains(ms, fmt.Sprintf("Date: %s", now.Format(time.RFC1123Z))) {
-		ea = append(ea, "Date")
-	}
-	if !strings.Contains(ms, `Message-ID: <message@id.com>`) {
-		ea = append(ea, "Message-ID")
-	}
-	if !strings.Contains(ms, `Precedence: bulk`) {
-		ea = append(ea, "Precedence")
-	}
-	if !strings.Contains(ms, `Subject: This is a subject`) {
-		ea = append(ea, "Subject")
-	}
-	if !strings.Contains(ms, `User-Agent: go-mail v`) {
-		ea = append(ea, "User-Agent")
-	}
-	if !strings.Contains(ms, `X-Mailer: go-mail v`) {
-		ea = append(ea, "X-Mailer")
-	}
-	if !strings.Contains(ms, `From: "Toni Tester" <test@example.com>`) {
-		ea = append(ea, "From")
-	}
-	if !strings.Contains(ms, `To: "Toni Receiver" <receiver@example.com>`) {
-		ea = append(ea, "To")
-	}
-	if !strings.Contains(ms, `Content-Type: text/plain; charset=UTF-8`) {
-		ea = append(ea, "Content-Type")
-	}
-	if !strings.Contains(ms, `Content-Transfer-Encoding: quoted-printable`) {
-		ea = append(ea, "Content-Transfer-Encoding")
-	}
-	if !strings.Contains(ms, "\r\n\r\nThis is the body") {
-		ea = append(ea, "Message body")
+		msgwriter.writer = buffer
+		message := testMessage(t)
+		message.SetDateWithValue(now)
+		message.SetMessageIDWithValue("message@id.com")
+		message.SetBulk()
+		msgwriter.writeMsg(message)
+		if msgwriter.err != nil {
+			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
 		}
 
-	pl := m.GetParts()
-	if len(pl) <= 0 {
-		t.Errorf("expected multiple parts but got none")
-		return
+		var incorrectFields []string
+		if !strings.Contains(buffer.String(), "MIME-Version: 1.0\r\n") {
+			incorrectFields = append(incorrectFields, "MIME-Version")
 		}
-	if len(pl) == 2 {
-		ap := pl[1]
-		ap.SetCharset(CharsetISO88591)
+		if !strings.Contains(buffer.String(), fmt.Sprintf("Date: %s\r\n", now.Format(time.RFC1123Z))) {
+			incorrectFields = append(incorrectFields, "Date")
 		}
-	buf.Reset()
-	mw.writeMsg(m)
-	ms = buf.String()
-	if !strings.Contains(ms, "\r\n\r\nThis is the alternative body") {
-		ea = append(ea, "Message alternative body")
+		if !strings.Contains(buffer.String(), "Message-ID: <message@id.com>\r\n") {
+			incorrectFields = append(incorrectFields, "Message-ID")
 		}
-	if !strings.Contains(ms, `Content-Type: text/html; charset=ISO-8859-1`) {
-		ea = append(ea, "alternative body charset")
+		if !strings.Contains(buffer.String(), "Precedence: bulk\r\n") {
+			incorrectFields = append(incorrectFields, "Precedence")
 		}
-
-	if len(ea) > 0 {
-		em := "writeMsg() failed. The following errors occurred:\n"
-		for e := range ea {
-			em += fmt.Sprintf("* incorrect %q field", ea[e])
+		if !strings.Contains(buffer.String(), "X-Auto-Response-Suppress: All\r\n") {
+			incorrectFields = append(incorrectFields, "X-Auto-Response-Suppress")
 		}
-		em += fmt.Sprintf("\n\nFull message:\n%s", ms)
-		t.Error(em)
+		if !strings.Contains(buffer.String(), "Subject: Testmail\r\n") {
+			incorrectFields = append(incorrectFields, "Subject")
 		}
+		if !strings.Contains(buffer.String(), "User-Agent: go-mail v") {
+			incorrectFields = append(incorrectFields, "User-Agent")
+		}
+		if !strings.Contains(buffer.String(), "X-Mailer: go-mail v") {
+			incorrectFields = append(incorrectFields, "X-Mailer")
+		}
+		if !strings.Contains(buffer.String(), `From: <`+TestSenderValid+`>`) {
+			incorrectFields = append(incorrectFields, "From")
+		}
+		if !strings.Contains(buffer.String(), `To: <`+TestRcptValid+`>`) {
+			incorrectFields = append(incorrectFields, "From")
+		}
+		if !strings.Contains(buffer.String(), "Content-Type: text/plain; charset=UTF-8\r\n") {
+			incorrectFields = append(incorrectFields, "Content-Type")
+		}
+		if !strings.Contains(buffer.String(), "Content-Transfer-Encoding: quoted-printable\r\n") {
+			incorrectFields = append(incorrectFields, "Content-Transfer-Encoding")
+		}
+		if !strings.HasSuffix(buffer.String(), "\r\n\r\nTestmail") {
+			incorrectFields = append(incorrectFields, "Message body")
+		}
+		if len(incorrectFields) > 0 {
+			t.Fatalf("msgWriter failed to write correct fields: %s - mail: %s",
+				strings.Join(incorrectFields, ", "), buffer.String())
+		}
+	})
+	t.Run("msgWriter with no from address uses envelope from", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("failed to create new message")
+		}
+		if err := message.EnvelopeFrom(TestSenderValid); err != nil {
+			t.Errorf("failed to set sender address: %s", err)
+		}
+		if err := message.To(TestRcptValid); err != nil {
+			t.Errorf("failed to set recipient address: %s", err)
+		}
+		message.Subject("Testmail")
+		message.SetBodyString(TypeTextPlain, "Testmail")
+		msgwriter.writeMsg(message)
+		if msgwriter.err != nil {
+			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
+		}
+		if !strings.Contains(buffer.String(), "From: <"+TestSenderValid+">") {
+			t.Errorf("expected envelope from address as from address, got: %s", buffer.String())
+		}
+	})
+	t.Run("msgWriter with no from address or envelope from", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("failed to create new message")
+		}
+		msgwriter.writeMsg(message)
+		if msgwriter.err != nil {
+			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
+		}
+		if strings.Contains(buffer.String(), "From:") {
+			t.Errorf("expected no from address, got: %s", buffer.String())
+		}
+	})
+	t.Run("msgWriter writes a multipart/mixed message", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		message := testMessage(t, WithBoundary("testboundary"))
+		message.AttachFile("testdata/attachment.txt")
+		msgwriter.writeMsg(message)
+		if msgwriter.err != nil {
+			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
+		}
+		if !strings.Contains(buffer.String(), "Content-Type: multipart/mixed") {
+			t.Errorf("expected multipart/mixed, got: %s", buffer.String())
+		}
+		if !strings.Contains(buffer.String(), "--testboundary\r\n") {
+			t.Errorf("expected boundary, got: %s", buffer.String())
+		}
+		if !strings.Contains(buffer.String(), "--testboundary--") {
+			t.Errorf("expected end boundary, got: %s", buffer.String())
+		}
+	})
+	t.Run("msgWriter writes a multipart/related message", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		message := testMessage(t, WithBoundary("testboundary"))
+		message.EmbedFile("testdata/embed.txt")
+		msgwriter.writeMsg(message)
+		if msgwriter.err != nil {
+			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
+		}
+		if !strings.Contains(buffer.String(), "Content-Type: multipart/related") {
+			t.Errorf("expected multipart/related, got: %s", buffer.String())
+		}
+		if !strings.Contains(buffer.String(), "--testboundary\r\n") {
+			t.Errorf("expected boundary, got: %s", buffer.String())
+		}
+		if !strings.Contains(buffer.String(), "--testboundary--") {
+			t.Errorf("expected end boundary, got: %s", buffer.String())
+		}
+	})
+	t.Run("msgWriter writes a multipart/alternative message", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		message := testMessage(t, WithBoundary("testboundary"))
+		message.AddAlternativeString(TypeTextHTML, "<html><body><h1>Testmail</h1></body></html>")
+		msgwriter.writeMsg(message)
+		if msgwriter.err != nil {
+			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
+		}
+		if !strings.Contains(buffer.String(), "Content-Type: multipart/alternative") {
+			t.Errorf("expected multipart/alternative, got: %s", buffer.String())
+		}
+		if !strings.Contains(buffer.String(), "--testboundary\r\n") {
+			t.Errorf("expected boundary, got: %s", buffer.String())
+		}
+		if !strings.Contains(buffer.String(), "--testboundary--") {
+			t.Errorf("expected end boundary, got: %s", buffer.String())
+		}
+	})
+	t.Run("msgWriter writes a application/pgp-encrypted message", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		message := testMessage(t, WithPGPType(PGPEncrypt), WithBoundary("testboundary"))
+		msgwriter.writeMsg(message)
+		if msgwriter.err != nil {
+			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
+		}
+		if !strings.Contains(buffer.String(), "Content-Type: multipart/encrypted") {
+			t.Errorf("expected multipart/encrypted, got: %s", buffer.String())
+		}
+		if !strings.Contains(buffer.String(), "--testboundary\r\n") {
+			t.Errorf("expected boundary, got: %s", buffer.String())
+		}
+	})
+	t.Run("msgWriter writes a application/pgp-signature message", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		message := testMessage(t, WithPGPType(PGPSignature), WithBoundary("testboundary"))
+		msgwriter.writeMsg(message)
+		if msgwriter.err != nil {
+			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
+		}
+		if !strings.Contains(buffer.String(), "Content-Type: multipart/signed") {
+			t.Errorf("expected multipart/signed, got: %s", buffer.String())
+		}
+		if !strings.Contains(buffer.String(), "--testboundary\r\n") {
+			t.Errorf("expected boundary, got: %s", buffer.String())
+		}
+	})
+	t.Run("msgWriter should ignore NoPGP", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		message := testMessage(t, WithBoundary("testboundary"))
+		message.pgptype = 9
+		msgwriter.writeMsg(message)
+		if msgwriter.err != nil {
+			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
+		}
+		if !strings.Contains(buffer.String(), "--testboundary\r\n") {
+			t.Errorf("expected boundary, got: %s", buffer.String())
+		}
+	})
 }
 
-// TestMsgWriter_writeMsg_PGP tests the writeMsg method of the msgWriter with PGP types set
-func TestMsgWriter_writeMsg_PGP(t *testing.T) {
-	m := NewMsg(WithPGPType(PGPEncrypt))
-	_ = m.From(`"Toni Tester" <test@example.com>`)
-	_ = m.To(`"Toni Receiver" <receiver@example.com>`)
-	m.Subject("This is a subject")
-	m.SetBodyString(TypeTextPlain, "This is the body")
-	buf := bytes.Buffer{}
-	mw := &msgWriter{writer: &buf, charset: CharsetUTF8, encoder: mime.QEncoding}
-	mw.writeMsg(m)
-	ms := buf.String()
-	if !strings.Contains(ms, `encrypted; protocol="application/pgp-encrypted"`) {
-		t.Errorf("writeMsg failed. Expected PGP encoding header but didn't find it in message output")
+func TestMsgWriter_writePreformattedGenHeader(t *testing.T) {
+	t.Run("message with no preformatted headerset", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter := &msgWriter{
+			writer:  buffer,
+			charset: CharsetUTF8,
+			encoder: getEncoder(EncodingQP),
 		}
-
-	m = NewMsg(WithPGPType(PGPSignature))
-	_ = m.From(`"Toni Tester" <test@example.com>`)
-	_ = m.To(`"Toni Receiver" <receiver@example.com>`)
-	m.Subject("This is a subject")
-	m.SetBodyString(TypeTextPlain, "This is the body")
-	buf = bytes.Buffer{}
-	mw = &msgWriter{writer: &buf, charset: CharsetUTF8, encoder: mime.QEncoding}
-	mw.writeMsg(m)
-	ms = buf.String()
-	if !strings.Contains(ms, `signed; protocol="application/pgp-signature"`) {
-		t.Errorf("writeMsg failed. Expected PGP encoding header but didn't find it in message output")
+		message := testMessage(t)
+		message.SetGenHeaderPreformatted(HeaderContentID, "This is a content id")
+		msgwriter.writeMsg(message)
+		if !strings.Contains(buffer.String(), "Content-ID: This is a content id\r\n") {
+			t.Errorf("expected preformatted header, got: %s", buffer.String())
 		}
+	})
+}
+
+func TestMsgWriter_addFiles(t *testing.T) {
+	msgwriter := &msgWriter{
+		charset: CharsetUTF8,
+		encoder: getEncoder(EncodingQP),
+	}
+	t.Run("message with a single file attached", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		message := testMessage(t)
+		message.AttachFile("testdata/attachment.txt")
+		msgwriter.writeMsg(message)
+		if msgwriter.err != nil {
+			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
+		}
+		switch runtime.GOOS {
+		case "windows":
+			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudA0K") {
+				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
+			}
+		default:
+			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudAo=") {
+				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
+			}
+		}
+		if !strings.Contains(buffer.String(), `Content-Disposition: attachment; filename="attachment.txt"`) {
+			t.Errorf("Content-Dispositon header not found for attachment. Mail: %s", buffer.String())
+		}
+		switch runtime.GOOS {
+		case "freebsd":
+			if !strings.Contains(buffer.String(), `Content-Type: application/octet-stream; name="attachment.txt"`) {
+				t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
+			}
+		default:
+			if !strings.Contains(buffer.String(), `Content-Type: text/plain; charset=utf-8; name="attachment.txt"`) {
+				t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
+			}
+		}
+	})
+	t.Run("message with a single file attached no extension", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		message := testMessage(t)
+		message.AttachFile("testdata/attachment")
+		msgwriter.writeMsg(message)
+		if msgwriter.err != nil {
+			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
+		}
+		switch runtime.GOOS {
+		case "windows":
+			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudA0K") {
+				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
+			}
+		default:
+			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudAo=") {
+				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
+			}
+		}
+		if !strings.Contains(buffer.String(), `Content-Disposition: attachment; filename="attachment"`) {
+			t.Errorf("Content-Dispositon header not found for attachment. Mail: %s", buffer.String())
+		}
+		if !strings.Contains(buffer.String(), `Content-Type: application/octet-stream; name="attachment"`) {
+			t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
+		}
+	})
+	t.Run("message with a single file attached custom content-type", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		message := testMessage(t)
+		message.AttachFile("testdata/attachment.txt", WithFileContentType(TypeAppOctetStream))
+		msgwriter.writeMsg(message)
+		if msgwriter.err != nil {
+			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
+		}
+		switch runtime.GOOS {
+		case "windows":
+			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudA0K") {
+				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
+			}
+		default:
+			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudAo=") {
+				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
+			}
+		}
+		if !strings.Contains(buffer.String(), `Content-Disposition: attachment; filename="attachment.txt"`) {
+			t.Errorf("Content-Dispositon header not found for attachment. Mail: %s", buffer.String())
+		}
+		if !strings.Contains(buffer.String(), `Content-Type: application/octet-stream; name="attachment.txt"`) {
+			t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
+		}
+	})
+	t.Run("message with a single file attached custom transfer-encoding", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		message := testMessage(t)
+		message.AttachFile("testdata/attachment.txt", WithFileEncoding(EncodingUSASCII))
+		msgwriter.writeMsg(message)
+		if msgwriter.err != nil {
+			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
+		}
+		if !strings.Contains(buffer.String(), "\r\n\r\nThis is a test attachment") {
+			t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
+		}
+		if !strings.Contains(buffer.String(), `Content-Disposition: attachment; filename="attachment.txt"`) {
+			t.Errorf("Content-Dispositon header not found for attachment. Mail: %s", buffer.String())
+		}
+		switch runtime.GOOS {
+		case "freebsd":
+			if !strings.Contains(buffer.String(), `Content-Type: application/octet-stream; name="attachment.txt"`) {
+				t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
+			}
+		default:
+			if !strings.Contains(buffer.String(), `Content-Type: text/plain; charset=utf-8; name="attachment.txt"`) {
+				t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
+			}
+		}
+		if !strings.Contains(buffer.String(), `Content-Transfer-Encoding: 7bit`) {
+			t.Errorf("Content-Transfer-Encoding header not found for attachment. Mail: %s", buffer.String())
+		}
+	})
+	t.Run("message with a single file attached custom description", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		message := testMessage(t)
+		message.AttachFile("testdata/attachment.txt", WithFileDescription("Testdescription"))
+		msgwriter.writeMsg(message)
+		if msgwriter.err != nil {
+			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
+		}
+		switch runtime.GOOS {
+		case "windows":
+			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudA0K") {
+				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
+			}
+		default:
+			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudAo=") {
+				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
+			}
+		}
+		if !strings.Contains(buffer.String(), `Content-Disposition: attachment; filename="attachment.txt"`) {
+			t.Errorf("Content-Dispositon header not found for attachment. Mail: %s", buffer.String())
+		}
+		switch runtime.GOOS {
+		case "freebsd":
+			if !strings.Contains(buffer.String(), `Content-Type: application/octet-stream; name="attachment.txt"`) {
+				t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
+			}
+		default:
+			if !strings.Contains(buffer.String(), `Content-Type: text/plain; charset=utf-8; name="attachment.txt"`) {
+				t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
+			}
+		}
+		if !strings.Contains(buffer.String(), `Content-Transfer-Encoding: base64`) {
+			t.Errorf("Content-Transfer-Encoding header not found for attachment. Mail: %s", buffer.String())
+		}
+		if !strings.Contains(buffer.String(), `Content-Description: Testdescription`) {
+			t.Errorf("Content-Description header not found for attachment. Mail: %s", buffer.String())
+		}
+	})
+	t.Run("message with attachment but no body part", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		message := testMessage(t)
+		message.parts = nil
+		message.AttachFile("testdata/attachment.txt")
+		msgwriter.writeMsg(message)
+		if msgwriter.err != nil {
+			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
+		}
+		switch runtime.GOOS {
+		case "windows":
+			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudA0K") {
+				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
+			}
+		default:
+			if !strings.Contains(buffer.String(), "VGhpcyBpcyBhIHRlc3QgYXR0YWNobWVudAo=") {
+				t.Errorf("attachment not found in mail message. Mail: %s", buffer.String())
+			}
+		}
+		if !strings.Contains(buffer.String(), `Content-Disposition: attachment; filename="attachment.txt"`) {
+			t.Errorf("Content-Dispositon header not found for attachment. Mail: %s", buffer.String())
+		}
+		switch runtime.GOOS {
+		case "freebsd":
+			if !strings.Contains(buffer.String(), `Content-Type: application/octet-stream; name="attachment.txt"`) {
+				t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
+			}
+		default:
+			if !strings.Contains(buffer.String(), `Content-Type: text/plain; charset=utf-8; name="attachment.txt"`) {
+				t.Errorf("Content-Type header not found for attachment. Mail: %s", buffer.String())
+			}
+		}
+		if !strings.Contains(buffer.String(), `Content-Transfer-Encoding: base64`) {
+			t.Errorf("Content-Transfer-Encoding header not found for attachment. Mail: %s", buffer.String())
+		}
+	})
+}
+
+func TestMsgWriter_writePart(t *testing.T) {
+	msgwriter := &msgWriter{
+		charset: CharsetUTF8,
+		encoder: getEncoder(EncodingQP),
+	}
+	t.Run("message with no part charset should use default message charset", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		message := testMessage(t, WithCharset(CharsetUTF7))
+		message.AddAlternativeString(TypeTextPlain, "thisisatest")
+		message.parts[1].charset = ""
+		msgwriter.writeMsg(message)
+		if msgwriter.err != nil {
+			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
+		}
+		if !strings.Contains(buffer.String(), "ontent-Type: text/plain; charset=UTF-7\r\n\r\nTestmail") {
+			t.Errorf("part not found in mail message. Mail: %s", buffer.String())
+		}
+		if !strings.Contains(buffer.String(), "ontent-Type: text/plain; charset=UTF-7\r\n\r\nthisisatest") {
+			t.Errorf("part not found in mail message. Mail: %s", buffer.String())
+		}
+	})
+	t.Run("message with parts that have a description", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		message := testMessage(t)
+		message.AddAlternativeString(TypeTextPlain, "thisisatest")
+		message.parts[1].description = "thisisadescription"
+		msgwriter.writeMsg(message)
+		if msgwriter.err != nil {
+			t.Errorf("msgWriter failed to write: %s", msgwriter.err)
+		}
+		if !strings.Contains(buffer.String(), "Content-Description: thisisadescription") {
+			t.Errorf("part description not found in mail message. Mail: %s", buffer.String())
+		}
+	})
+}
+
+func TestMsgWriter_writeString(t *testing.T) {
+	msgwriter := &msgWriter{
+		charset: CharsetUTF8,
+		encoder: getEncoder(EncodingQP),
+	}
+	t.Run("writeString succeeds", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		msgwriter.writeString("thisisatest")
+		if !strings.EqualFold(buffer.String(), "thisisatest") {
+			t.Errorf("writeString failed, expected: thisisatest got: %s", buffer.String())
+		}
+	})
+	t.Run("writeString fails", func(t *testing.T) {
+		msgwriter.writer = failReadWriteSeekCloser{}
+		msgwriter.writeString("thisisatest")
+		if msgwriter.err == nil {
+			t.Errorf("writeString succeeded, expected error")
+		}
+	})
+	t.Run("writeString on errored writer should return", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		msgwriter.err = errors.New("intentional error")
+		msgwriter.writeString("thisisatest")
+		if !strings.EqualFold(buffer.String(), "") {
+			t.Errorf("writeString succeeded, expected: empty string, got: %s", buffer.String())
+		}
+	})
+}
+
+func TestMsgWriter_writeHeader(t *testing.T) {
+	msgwriter := &msgWriter{
+		charset: CharsetUTF8,
+		encoder: getEncoder(EncodingQP),
+	}
+	t.Run("writeHeader with single value", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		msgwriter.writeHeader(HeaderMessageID, "this.is.a.test")
+		if !strings.EqualFold(buffer.String(), "Message-ID: this.is.a.test\r\n") {
+			t.Errorf("writeHeader failed, expected: %s, got: %s", "Message-ID: this.is.a.test",
+				buffer.String())
+		}
+	})
+	t.Run("writeHeader with multiple values", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		msgwriter.writeHeader(HeaderMessageID, "this.is.a.test", "this.as.well")
+		if !strings.EqualFold(buffer.String(), "Message-ID: this.is.a.test, this.as.well\r\n") {
+			t.Errorf("writeHeader failed, expected: %s, got: %s", "Message-ID: this.is.a.test, this.as.well",
+				buffer.String())
+		}
+	})
+	t.Run("writeHeader with no values", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		msgwriter.writeHeader(HeaderMessageID)
+		// While technically it is permitted to have empty headers, it's recommend to omit them if
+		// no value is present. We follow this recommendation.
+		if !strings.EqualFold(buffer.String(), "") {
+			t.Errorf("writeHeader failed, expected: %s, got: %s", "", buffer.String())
+		}
+	})
+	t.Run("writeHeader with very long value", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		msgwriter.writeHeader(HeaderMessageID, strings.Repeat("a", MaxHeaderLength-13), "next-row")
+		want := "Message-ID:\r\n " + strings.Repeat("a", MaxHeaderLength-13) + ",\r\n next-row\r\n"
+		if !strings.EqualFold(buffer.String(), want) {
+			t.Errorf("writeHeader failed, expected: %s, got: %s", want, buffer.String())
+		}
+	})
+}
+
+func TestMsgWriter_writeBody(t *testing.T) {
+	t.Log("We only cover some edge-cases here, most of the functionality is tested already very thoroughly.")
+
+	msgwriter := &msgWriter{
+		charset: CharsetUTF8,
+		encoder: getEncoder(EncodingQP),
+	}
+	t.Run("writeBody on NoEncoding", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		message := testMessage(t)
+		msgwriter.writeBody(message.parts[0].writeFunc, NoEncoding)
+		if msgwriter.err != nil {
+			t.Errorf("writeBody failed to write: %s", msgwriter.err)
+		}
+	})
+	t.Run("writeBody on NoEncoding fails on write", func(t *testing.T) {
+		msgwriter.writer = failReadWriteSeekCloser{}
+		message := testMessage(t)
+		msgwriter.writeBody(message.parts[0].writeFunc, NoEncoding)
+		if msgwriter.err == nil {
+			t.Errorf("writeBody succeeded, expected error")
+		}
+		if !strings.EqualFold(msgwriter.err.Error(), "bodyWriter io.Copy: intentional write failure") {
+			t.Errorf("expected error: bodyWriter io.Copy: intentional write failure, got: %s", msgwriter.err)
+		}
+	})
+	t.Run("writeBody on NoEncoding fails on writeFunc", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		writeFunc := func(io.Writer) (int64, error) {
+			return 0, errors.New("intentional write failure")
+		}
+		msgwriter.writeBody(writeFunc, NoEncoding)
+		if msgwriter.err == nil {
+			t.Errorf("writeBody succeeded, expected error")
+		}
+		if !strings.EqualFold(msgwriter.err.Error(), "bodyWriter function: intentional write failure") {
+			t.Errorf("expected error: bodyWriter function: intentional write failure, got: %s", msgwriter.err)
+		}
+	})
+	t.Run("writeBody Quoted-Printable fails on write", func(t *testing.T) {
+		msgwriter.writer = failReadWriteSeekCloser{}
+		message := testMessage(t)
+		msgwriter.writeBody(message.parts[0].writeFunc, EncodingQP)
+		if msgwriter.err == nil {
+			t.Errorf("writeBody succeeded, expected error")
+		}
+		if !strings.EqualFold(msgwriter.err.Error(), "bodyWriter function: intentional write failure") {
+			t.Errorf("expected error: bodyWriter function: intentional write failure, got: %s", msgwriter.err)
+		}
+	})
+	t.Run("writeBody Quoted-Printable fails on writeFunc", func(t *testing.T) {
+		buffer := bytes.NewBuffer(nil)
+		msgwriter.writer = buffer
+		writeFunc := func(io.Writer) (int64, error) {
+			return 0, errors.New("intentional write failure")
+		}
+		msgwriter.writeBody(writeFunc, EncodingQP)
+		if msgwriter.err == nil {
+			t.Errorf("writeBody succeeded, expected error")
+		}
+		if !strings.EqualFold(msgwriter.err.Error(), "bodyWriter function: intentional write failure") {
+			t.Errorf("expected error: bodyWriter function: intentional write failure, got: %s", msgwriter.err)
+		}
+	})
 }

testdata/attachment

@@ -0,0 +1 @@
+This is a test attachment

testdata/attachment.license

@@ -0,0 +1,3 @@
+// SPDX-FileCopyrightText: Copyright (c) 2022-2024 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT