Update user agent test in msg_test.go

Updated the test "check default user agent" in `msg_test.go` to reflect dynamic versioning. The wantUserAgent field now uses fmt.Sprintf to combine the go-mail version dynamically, improving the accuracy of testing.

msg.go

@@ -117,6 +117,9 @@ type Msg struct {
 
 	// sendError holds the SendError in case a Msg could not be delivered during the Client.Send operation
 	sendError error
+
+	// noDefaultUserAgent indicates whether the default User Agent will be excluded for the Msg when it's sent.
+	noDefaultUserAgent bool
 }
 
 // SendmailPath is the default system path to the sendmail binary
@@ -192,6 +195,13 @@ func WithPGPType(pt PGPType) MsgOption {
 	}
 }
 
+// WithNoDefaultUserAgent configures the Msg to not use the default User Agent
+func WithNoDefaultUserAgent() MsgOption {
+	return func(m *Msg) {
+		m.noDefaultUserAgent = true
+	}
+}
+
 // SetCharset sets the encoding charset of the Msg
 func (m *Msg) SetCharset(c Charset) {
 	m.charset = c
@@ -1181,6 +1191,9 @@ func (m *Msg) setEncoder() {
 // checkUserAgent checks if a useragent/x-mailer is set and if not will set a default
 // version string
 func (m *Msg) checkUserAgent() {
+	if m.noDefaultUserAgent {
+		return
+	}
 	_, uaok := m.genHeader[HeaderUserAgent]
 	_, xmok := m.genHeader[HeaderXMailer]
 	if !uaok && !xmok {

msg_test.go

@@ -3161,3 +3161,61 @@ func TestMsg_BccFromString(t *testing.T) {
 		})
 	}
 }
+
+// TestMsg_checkUserAgent tests the checkUserAgent method of the Msg
+func TestMsg_checkUserAgent(t *testing.T) {
+	tests := []struct {
+		name               string
+		noDefaultUserAgent bool
+		genHeader          map[Header][]string
+		wantUserAgent      string
+		sf                 bool
+	}{
+		{
+			name:               "check default user agent",
+			noDefaultUserAgent: false,
+			wantUserAgent:      fmt.Sprintf("go-mail v%s // https://github.com/wneessen/go-mail", VERSION),
+			sf:                 false,
+		},
+		{
+			name:               "check no default user agent",
+			noDefaultUserAgent: true,
+			wantUserAgent:      "",
+			sf:                 true,
+		},
+		{
+			name:               "check if ua and xm is already set",
+			noDefaultUserAgent: false,
+			genHeader: map[Header][]string{
+				HeaderUserAgent: {"custom UA"},
+				HeaderXMailer:   {"custom XM"},
+			},
+			wantUserAgent: "custom UA",
+			sf:            false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			msg := &Msg{
+				noDefaultUserAgent: tt.noDefaultUserAgent,
+				genHeader:          tt.genHeader,
+			}
+			msg.checkUserAgent()
+			gotUserAgent := ""
+			if val, ok := msg.genHeader[HeaderUserAgent]; ok {
+				gotUserAgent = val[0] // Assuming the first one is the needed value
+			}
+			if gotUserAgent != tt.wantUserAgent && !tt.sf {
+				t.Errorf("UserAgent got = %v, want = %v", gotUserAgent, tt.wantUserAgent)
+			}
+		})
+	}
+}
+
+// TestNewMsgWithMIMEVersion tests WithMIMEVersion and Msg.SetMIMEVersion
+func TestNewMsgWithNoDefaultUserAgent(t *testing.T) {
+	m := NewMsg(WithNoDefaultUserAgent())
+	if m.noDefaultUserAgent != true {
+		t.Errorf("WithNoDefaultUserAgent() failed. Expected: %t, got: %t", true, false)
+	}
+}