When decoding a multipart message from EML, use the original EML boundary instead of generating a new one

eml.go

@@ -270,6 +270,9 @@ func parseEMLBodyParts(parsedMsg *netmail.Message, bodybuf *bytes.Buffer, msg *M
 	if value, ok := params["charset"]; ok {
 		msg.SetCharset(Charset(value))
 	}
+	if value, ok := params["boundary"]; ok {
+		msg.SetBoundary(value)
+	}
 
 	switch {
 	case strings.EqualFold(mediatype, TypeTextPlain.String()),

eml_test.go

@@ -1231,3 +1231,30 @@ func TestEMLToMsgFromFile(t *testing.T) {
 		}
 	})
 }
+
+func TestToAndFromEMLMultipart(t *testing.T) {
+	t.Run("TestToAndFromEMLMultipart succeeds", func(t *testing.T) {
+		msg := NewMsg()
+		_ = msg.To("[email protected]")
+		_ = msg.FromFormat("Person", "[email protected]")
+		msg.Subject("Test Subject")
+
+		msg.SetBodyString(TypeTextPlain, "hi")
+		msg.AddAlternativeString(TypeTextHTML, "<h1>hi</h1>")
+
+		var emlBuf bytes.Buffer
+		_, _ = msg.WriteTo(&emlBuf)
+
+		decMsg, _ := EMLToMsgFromReader(&emlBuf)
+
+		var resBuf bytes.Buffer
+		_, _ = decMsg.WriteTo(&resBuf)
+
+		str := resBuf.String()
+
+		contentTypeCount := strings.Count(str, "Content-Type: multipart/alternative")
+		if contentTypeCount != 1 {
+			t.Fatalf("TestToAndFromEMLMultipart failed: want 1 multipart/alternative Content-Type header, got %d", contentTypeCount)
+		}
+	})
+}

msgwriter.go

@@ -137,28 +137,28 @@ func (mw *msgWriter) writeMsg(msg *Msg) {
 			mw.err = err
 			return
 		}
-		mw.startMP(MIMESMIMESigned, boundary)
+		mw.startMP(msg, MIMESMIMESigned, boundary)
 		mw.writeString(DoubleNewLine)
 	}
 	if msg.hasMixed() {
 		boundary := mw.getMultipartBoundary(msg, MIMEMixed)
-		boundary = mw.startMP(MIMEMixed, boundary)
+		boundary = mw.startMP(msg, MIMEMixed, boundary)
 		msg.multiPartBoundary[MIMEMixed] = boundary
 		if mw.depth == 1 {
 			mw.writeString(DoubleNewLine)
 		}
 	}
 	if msg.hasRelated() {
 		boundary := mw.getMultipartBoundary(msg, MIMERelated)
-		boundary = mw.startMP(MIMERelated, boundary)
+		boundary = mw.startMP(msg, MIMERelated, boundary)
 		msg.multiPartBoundary[MIMERelated] = boundary
 		if mw.depth == 1 {
 			mw.writeString(DoubleNewLine)
 		}
 	}
 	if msg.hasAlt() {
 		boundary := mw.getMultipartBoundary(msg, MIMEAlternative)
-		boundary = mw.startMP(MIMEAlternative, boundary)
+		boundary = mw.startMP(msg, MIMEAlternative, boundary)
 		msg.multiPartBoundary[MIMEAlternative] = boundary
 		if mw.depth == 1 {
 			mw.writeString(DoubleNewLine)
@@ -167,10 +167,10 @@ func (mw *msgWriter) writeMsg(msg *Msg) {
 	if msg.hasPGPType() {
 		switch msg.pgptype {
 		case PGPEncrypt:
-			mw.startMP(`encrypted; protocol="application/pgp-encrypted"`,
+			mw.startMP(msg, `encrypted; protocol="application/pgp-encrypted"`,
 				msg.boundary)
 		case PGPSignature:
-			mw.startMP(`signed; protocol="application/pgp-signature";`,
+			mw.startMP(msg, `signed; protocol="application/pgp-signature";`,
 				msg.boundary)
 		default:
 		}
@@ -251,6 +251,7 @@ func (mw *msgWriter) writePreformattedGenHeader(msg *Msg) {
 // generated. It also handles writing a new part when nested multipart structures are used.
 //
 // Parameters:
+//   - msg: The message whose multipart headers are being written.
 //   - mimeType: The MIME type of the multipart content (e.g., "mixed", "alternative").
 //   - boundary: The boundary string separating different parts of the multipart message.
 //
@@ -259,7 +260,7 @@ func (mw *msgWriter) writePreformattedGenHeader(msg *Msg) {
 //
 // References:
 //   - https://datatracker.ietf.org/doc/html/rfc2046
-func (mw *msgWriter) startMP(mimeType MIMEType, boundary string) string {
+func (mw *msgWriter) startMP(msg *Msg, mimeType MIMEType, boundary string) string {
 	multiPartWriter := multipart.NewWriter(mw)
 	if boundary != "" {
 		mw.err = multiPartWriter.SetBoundary(boundary)
@@ -269,12 +270,16 @@ func (mw *msgWriter) startMP(mimeType MIMEType, boundary string) string {
 		multiPartWriter.Boundary())
 	mw.multiPartWriter[mw.depth] = multiPartWriter
 
-	if mw.depth == 0 {
-		mw.writeString(fmt.Sprintf("%s: %s", HeaderContentType, contentType))
-	}
-	if mw.depth > 0 {
-		mw.newPart(map[string][]string{"Content-Type": {contentType}})
+	// Do not write Content-Type if the header was already written as part of genHeaders.
+	if _, ok := msg.genHeader[HeaderContentType]; !ok {
+		if mw.depth == 0 {
+			mw.writeString(fmt.Sprintf("%s: %s", HeaderContentType, contentType))
+		}
+		if mw.depth > 0 {
+			mw.newPart(map[string][]string{"Content-Type": {contentType}})
+		}
 	}
+
 	mw.depth++
 	return multiPartWriter.Boundary()
 }