Add tests for SendError MessageID and Msg methods

Introduce two new tests in senderror_test.go: TestSendError_MessageID and TestSendError_Msg. These tests validate the behavior of the MessageID and Msg methods of the SendError type, ensuring correct handling of message ID and sender information.

senderror_test.go

@@ -90,7 +90,55 @@ func TestSendError_IsTempNil(t *testing.T) {
 	}
 }
 
+func TestSendError_MessageID(t *testing.T) {
+	var se *SendError
+	err := returnSendError(ErrAmbiguous, false)
+	if !errors.As(err, &se) {
+		t.Errorf("error mismatch, expected error to be of type *SendError")
+		return
+	}
+	if errors.As(err, &se) {
+		if se.MessageID() == "" {
+			t.Errorf("sendError expected message-id, but got empty string")
+		}
+		if !strings.EqualFold(se.MessageID(), "<this.is.a.message.id>") {
+			t.Errorf("sendError message-id expected: %s, but got: %s", "<this.is.a.message.id>",
+				se.MessageID())
+		}
+	}
+}
+
+func TestSendError_Msg(t *testing.T) {
+	var se *SendError
+	err := returnSendError(ErrAmbiguous, false)
+	if !errors.As(err, &se) {
+		t.Errorf("error mismatch, expected error to be of type *SendError")
+		return
+	}
+	if errors.As(err, &se) {
+		if se.Msg() == nil {
+			t.Errorf("sendError expected msg pointer, but got nil")
+		}
+		from := se.Msg().GetFromString()
+		if len(from) == 0 {
+			t.Errorf("sendError expected msg from, but got empty string")
+			return
+		}
+		if !strings.EqualFold(from[0], "<toni.tester@domain.tld>") {
+			t.Errorf("sendError message from expected: %s, but got: %s", "<toni.tester@domain.tld>",
+				from[0])
+		}
+	}
+}
+
 // returnSendError is a helper method to retunr a SendError with a specific reason
 func returnSendError(r SendErrReason, t bool) error {
-	return &SendError{Reason: r, isTemp: t}
+	message := NewMsg()
+	_ = message.From("toni.tester@domain.tld")
+	_ = message.To("tina.tester@domain.tld")
+	message.Subject("This is the subject")
+	message.SetBodyString(TypeTextPlain, "This is the message body")
+	message.SetMessageIDWithValue("this.is.a.message.id")
+
+	return &SendError{Reason: r, isTemp: t, affectedMsg: message}
 }