diff --git a/client_test.go b/client_test.go
index 8408035..1ba0fc9 100644
--- a/client_test.go
+++ b/client_test.go
@@ -3528,9 +3528,9 @@ 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")
 	}
diff --git a/msgwriter_test.go b/msgwriter_test.go
index a41e5d3..babb52a 100644
--- a/msgwriter_test.go
+++ b/msgwriter_test.go
@@ -6,151 +6,293 @@ package mail
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
-	"io"
 	"mime"
 	"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")
-	}
-
-	// 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")
-	}
-}
-
-// 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()
-	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")
-	}
-
-	pl := m.GetParts()
-	if len(pl) <= 0 {
-		t.Errorf("expected multiple parts but got none")
-		return
-	}
-	if len(pl) == 2 {
-		ap := pl[1]
-		ap.SetCharset(CharsetISO88591)
-	}
-	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(ms, `Content-Type: text/html; charset=ISO-8859-1`) {
-		ea = append(ea, "alternative body charset")
-	}
-
-	if len(ea) > 0 {
-		em := "writeMsg() failed. The following errors occurred:\n"
-		for e := range ea {
-			em += fmt.Sprintf("* incorrect %q field", ea[e])
+	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,
+				}
+				_, err := msgwriter.Write([]byte("test"))
+				if err != nil {
+					t.Errorf("msgWriter failed to write: %s", err)
+				}
+			})
 		}
-		em += fmt.Sprintf("\n\nFull message:\n%s", ms)
-		t.Error(em)
-	}
+	})
+	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_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_writeMsg(t *testing.T) {
+	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()
+		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)
+		}
 
-	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")
-	}
+		var incorrectFields []string
+		if !strings.Contains(buffer.String(), "MIME-Version: 1.0\r\n") {
+			incorrectFields = append(incorrectFields, "MIME-Version")
+		}
+		if !strings.Contains(buffer.String(), fmt.Sprintf("Date: %s\r\n", now.Format(time.RFC1123Z))) {
+			incorrectFields = append(incorrectFields, "Date")
+		}
+		if !strings.Contains(buffer.String(), "Message-ID: <message@id.com>\r\n") {
+			incorrectFields = append(incorrectFields, "Message-ID")
+		}
+		if !strings.Contains(buffer.String(), "Precedence: bulk\r\n") {
+			incorrectFields = append(incorrectFields, "Precedence")
+		}
+		if !strings.Contains(buffer.String(), "X-Auto-Response-Suppress: All\r\n") {
+			incorrectFields = append(incorrectFields, "X-Auto-Response-Suppress")
+		}
+		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())
+		}
+	})
+}
+
+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),
+		}
+		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())
+		}
+	})
 }