Closes #11

This PR adds a `Msg.Read()` method which satisfies the `io.Reader` interface

README.md

@@ -32,8 +32,9 @@ Some of the features of this library:
 * [X] Support for common mail header field generation (Message-ID, Date, Bulk-Precedence, Priority, etc.)
 * [X] Reusing the same SMTP connection to send multiple mails
 * [X] Support for attachments and inline embeds
-* [X] Support for different encodings (existing but not fully tested)
+* [X] Support for different encodings
 * [X] Support sending mails via a local sendmail command
+* [X] Message object satisfies `io.WriteTo` and `io.Reader` interfaces
 
 go-mail works like a programatic email client and provides lots of methods and functionalities you would consider
 standard in a MUA.

msg.go

@@ -552,6 +552,16 @@ func (m *Msg) WriteToSendmailWithContext(ctx context.Context, sp string, a ...st
 	return nil
 }
 
+// Read outputs the length of p into p to satisfy the io.Reader interface
+func (m *Msg) Read(p []byte) (int, error) {
+	wbuf := bytes.Buffer{}
+	_, err := m.WriteTo(&wbuf)
+	if err != nil {
+		return 0, fmt.Errorf("failed to write message to internal write buffer: %w", err)
+	}
+	return wbuf.Read(p)
+}
+
 // encodeString encodes a string based on the configured message encoder and the corresponding
 // charset for the Msg
 func (m *Msg) encodeString(s string) string {

msg_test.go

@@ -1272,3 +1272,69 @@ func TestMsg_multipleWrites(t *testing.T) {
 		t.Errorf("second WriteTo() body does not contain unique string: %s", ts)
 	}
 }
+
+// TestMsg_Read tests the Msg.Read method that implements the io.Reader interface
+func TestMsg_Read(t *testing.T) {
+	tests := []struct {
+		name string
+		plen int
+	}{
+		{"P length is bigger than the mail", 32000},
+		{"P length is smaller than the mail", 128},
+	}
+
+	m := NewMsg()
+	m.SetBodyString(TypeTextPlain, "TEST123")
+	wbuf := bytes.Buffer{}
+	_, err := m.Write(&wbuf)
+	if err != nil {
+		t.Errorf("failed to write message into temporary buffer: %s", err)
+	}
+	elen := wbuf.Len()
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			p := make([]byte, tt.plen)
+			n, err := m.Read(p)
+			if err != nil {
+				t.Errorf("failed to Read(): %s", err)
+			}
+			if n == 0 {
+				t.Errorf("failed to Read() - received 0 bytes of data")
+			}
+			if tt.plen >= elen && n != elen {
+				t.Errorf("failed to Read() - not all data received. Expected: %d, got: %d", elen, n)
+			}
+			if tt.plen < elen && n != tt.plen {
+				t.Errorf("failed to Read() - full length of p wasn't filled with data. Expected: %d, got: %d",
+					tt.plen, n)
+			}
+		})
+	}
+}
+
+// TestMsg_Read_ioCopy tests the Msg.Read method using io.Copy
+func TestMsg_Read_ioCopy(t *testing.T) {
+	wbuf1 := bytes.Buffer{}
+	wbuf2 := bytes.Buffer{}
+	m := NewMsg()
+	m.SetBodyString(TypeTextPlain, "TEST123")
+
+	// First we use WriteTo to have something to compare to
+	_, err := m.WriteTo(&wbuf1)
+	if err != nil {
+		t.Errorf("failed to write body to buffer: %s", err)
+	}
+
+	// Then we write to wbuf2 via io.Copy
+	n, err := io.Copy(&wbuf2, m)
+	if err != nil {
+		t.Errorf("failed to use io.Copy on Msg: %s", err)
+	}
+	if n != int64(wbuf1.Len()) {
+		t.Errorf("message length of WriteTo and io.Copy differ. Expected: %d, got: %d", wbuf1.Len(), n)
+	}
+	if wbuf1.String() != wbuf2.String() {
+		t.Errorf("message content of WriteTo and io.Copy differ")
+	}
+}