Add tests for SetMessageID and GetMessageID methods

Introduce unit tests to ensure SetMessageID generates unique IDs and GetMessageID correctly formats various input IDs. Additionally, verify GetMessageID returns an empty string when no ID is set.

msg_test.go

@@ -1601,6 +1601,67 @@ func TestMsg_Subject(t *testing.T) {
 	}
 }
 
+func TestMsg_SetMessageID(t *testing.T) {
+	t.Run("SetMessageID randomness", func(t *testing.T) {
+		var mids []string
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		for i := 0; i < 50_000; i++ {
+			message.SetMessageID()
+			mid := message.GetMessageID()
+			mids = append(mids, mid)
+		}
+		c := make(map[string]int)
+		for i := range mids {
+			c[mids[i]]++
+		}
+		for k, v := range c {
+			if v > 1 {
+				t.Errorf("MessageID randomness not given. MessageID %q was generated %d times", k, v)
+			}
+		}
+	})
+}
+func TestMsg_GetMessageID(t *testing.T) {
+	t.Run("GetMessageID with normal IDs", func(t *testing.T) {
+		tests := []struct {
+			msgid string
+			want  string
+		}{
+			{"this.is.a.test", "<this.is.a.test>"},
+			{"12345.6789@domain.com", "<12345.6789@domain.com>"},
+			{"abcd1234@sub.domain.com", "<abcd1234@sub.domain.com>"},
+			{"uniqeID-123@domain.co.tld", "<uniqeID-123@domain.co.tld>"},
+			{"2024_10_26192300@domain.tld", "<2024_10_26192300@domain.tld>"},
+		}
+		for _, tt := range tests {
+			t.Run(tt.msgid, func(t *testing.T) {
+				message := NewMsg()
+				if message == nil {
+					t.Fatal("message is nil")
+				}
+				message.SetMessageIDWithValue(tt.msgid)
+				msgid := message.GetMessageID()
+				if !strings.EqualFold(tt.want, msgid) {
+					t.Errorf("GetMessageID() failed. Want: %s, got: %s", tt.want, msgid)
+				}
+			})
+		}
+	})
+	t.Run("GetMessageID no messageID set should return an empty string", func(t *testing.T) {
+		message := NewMsg()
+		if message == nil {
+			t.Fatal("message is nil")
+		}
+		msgid := message.GetMessageID()
+		if msgid != "" {
+			t.Errorf("GetMessageID() failed. Want: empty string, got: %s", msgid)
+		}
+	})
+}
+
 // checkAddrHeader verifies the correctness of an AddrHeader in a Msg based on the provided criteria.
 // It checks whether the AddrHeader contains the correct address, name, and number of fields.
 func checkAddrHeader(t *testing.T, message *Msg, header AddrHeader, fn string, field, wantFields int,