Add SetHeaderPreformatted() method

With the SetHeaderPreformatted() method we have the ability to set headers that are already preformatted by the user and will not be altered in the mail message output

msg.go

@@ -77,6 +77,11 @@ type Msg struct {
 	// genHeader is a slice of strings that the different generic mail Header fields
 	genHeader map[Header][]string
 
+	// preformHeader is a slice of strings that the different generic mail Header fields
+	// of which content is already preformated and will not be affected by the automatic line
+	// breaks
+	preformHeader map[Header]string
+
 	// mimever represents the MIME version
 	mimever MIMEVersion
 
@@ -100,6 +105,7 @@ func NewMsg(o ...MsgOption) *Msg {
 		charset:       CharsetUTF8,
 		encoding:      EncodingQP,
 		genHeader:     make(map[Header][]string),
+		preformHeader: make(map[Header]string),
 		mimever:       Mime10,
 	}
 
@@ -194,6 +200,24 @@ func (m *Msg) SetHeader(h Header, v ...string) {
 	m.genHeader[h] = v
 }
 
+// SetHeaderPreformatted sets a generic header field of the Msg which content is
+// already preformated.
+//
+// This method does not take a slice of values but only a single value. This is
+// due to the fact, that we do not perform any content alteration and expect the
+// user has already done so
+//
+// **Please note:** This method should be used only as a last resort. Since the
+// user is respondible for the formating of the message header, go-mail cannot
+// guarantee the fully compliance with the RFC 2822. It is recommended to use
+// SetHeader instead.
+func (m *Msg) SetHeaderPreformatted(h Header, v string) {
+	if m.preformHeader == nil {
+		m.preformHeader = make(map[Header]string)
+	}
+	m.preformHeader[h] = v
+}
+
 // SetAddrHeader sets an address related header field of the Msg
 func (m *Msg) SetAddrHeader(h AddrHeader, v ...string) error {
 	if m.addrHeader == nil {

msg_test.go

@@ -255,7 +255,7 @@ func TestApplyMiddlewares(t *testing.T) {
 	}
 }
 
-// TestMsg_SetHEader tests Msg.SetHeader
+// TestMsg_SetHeader tests Msg.SetHeader
 func TestMsg_SetHeader(t *testing.T) {
 	tests := []struct {
 		name   string
@@ -289,6 +289,46 @@ func TestMsg_SetHeader(t *testing.T) {
 	}
 }
 
+// TestMsg_SetHeaderPreformatted tests Msg.SetHeaderPreformatted
+func TestMsg_SetHeaderPreformatted(t *testing.T) {
+	tests := []struct {
+		name   string
+		header Header
+		value  string
+	}{
+		{"set subject", HeaderSubject, "This is Subject"},
+		{"set content-language", HeaderContentLang, fmt.Sprintf("%s, %s, %s, %s",
+			"en", "de", "fr", "es")},
+		{"set subject with newline", HeaderSubject, "This is Subject\r\n with 2nd line"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			m := &Msg{}
+			m.SetHeaderPreformatted(tt.header, tt.value)
+			m = NewMsg()
+			m.SetHeaderPreformatted(tt.header, tt.value)
+			if m.preformHeader[tt.header] == "" {
+				t.Errorf("SetHeaderPreformatted() failed. Tried to set header %s, but it is empty", tt.header)
+			}
+			if m.preformHeader[tt.header] != tt.value {
+				t.Errorf("SetHeaderPreformatted() failed. Expected: %q, got: %q", tt.value,
+					m.preformHeader[tt.header])
+			}
+			buf := bytes.Buffer{}
+			_, err := m.WriteTo(&buf)
+			if err != nil {
+				t.Errorf("failed to write message to memory: %s", err)
+				return
+			}
+			if !strings.Contains(buf.String(), fmt.Sprintf("%s: %s%s", tt.header, tt.value, SingleNewLine)) {
+				t.Errorf("SetHeaderPreformatted() failed. Unable to find correctly formated header in " +
+					"mail message output")
+			}
+		})
+	}
+}
+
 // TestMsg_AddTo tests the Msg.AddTo method
 func TestMsg_AddTo(t *testing.T) {
 	a := []string{"address1@example.com", "address2@example.com"}

msgwriter.go

@@ -62,6 +62,7 @@ func (mw *msgWriter) writeMsg(m *Msg) {
 	m.addDefaultHeader()
 	m.checkUserAgent()
 	mw.writeGenHeader(m)
+	mw.writePreformattedGenHeader(m)
 
 	// Set the FROM header (or envelope FROM if FROM is empty)
 	hf := true
@@ -132,6 +133,13 @@ func (mw *msgWriter) writeGenHeader(m *Msg) {
 	}
 }
 
+// writePreformatedHeader writes out all preformated generic headers to the msgWriter
+func (mw *msgWriter) writePreformattedGenHeader(m *Msg) {
+	for k, v := range m.preformHeader {
+		mw.writeString(fmt.Sprintf("%s: %s%s", k, v, SingleNewLine))
+	}
+}
+
 // startMP writes a multipart beginning
 func (mw *msgWriter) startMP(mt MIMEType, b string) {
 	mp := multipart.NewWriter(mw)