Simplify Message-ID generation

Updated the SetMessageID method to generate a "Message-ID" using a
single randomly generated string combined with the hostname, replacing
the prior complex format that included process ID and multiple random
numbers. This change simplifies the code of the generated IDs.

msg.go

@@ -972,12 +972,12 @@ func (m *Msg) Subject(subj string) {
 
 // SetMessageID generates and sets a unique "Message-ID" header for the Msg.
 //
-// This method creates a "Message-ID" string using the current process ID, random numbers, and the hostname
-// of the machine. The generated ID helps uniquely identify the message in email systems, facilitating tracking
-// and preventing duplication. If the hostname cannot be retrieved, it defaults to "localhost.localdomain".
+// This method creates a "Message-ID" string using a randomly generated string and the hostname of the machine.
+// The generated ID helps uniquely identify the message in email systems, facilitating tracking and preventing
+// duplication. If the hostname cannot be retrieved, it defaults to "localhost.localdomain".
 //
 // The generated Message-ID follows the format
-// "<processID.randomNumberPrimary.randomNumberSecondary.randomString@hostname>".
+// "<randomString@hostname>".
 //
 // References:
 // - https://datatracker.ietf.org/doc/html/rfc5322#section-3.6.4
@@ -986,13 +986,9 @@ func (m *Msg) SetMessageID() {
  if err != nil {
  hostname = "localhost.localdomain"
  }
- randNumPrimary := randNum(100000000)
- randNumSecondary := randNum(10000)
- randString, _ := randomStringSecure(17)
- procID := os.Getpid() * randNumSecondary
- messageID := fmt.Sprintf("%d.%d%d.%s@%s", procID, randNumPrimary, randNumSecondary,
- randString, hostname)
- m.SetMessageIDWithValue(messageID)
+ // We have 64 possible characters, which for a 22 character string, provides approx. 132 bits of entropy.
+ randString, _ := randomStringSecure(22)
+ m.SetMessageIDWithValue(fmt.Sprintf("%s@%s", randString, hostname))
 }
 
 // GetMessageID retrieves the "Message-ID" header from the Msg.

msg_test.go

@@ -786,8 +786,8 @@ func TestMsg_SetMessageIDWithValue(t *testing.T) {
 // TestMsg_SetMessageIDRandomness tests the randomness of Msg.SetMessageID methods
 func TestMsg_SetMessageIDRandomness(t *testing.T) {
  var mids []string
+ m := NewMsg()
  for i := 0; i < 50_000; i++ {
- m := NewMsg()
  m.SetMessageID()
  mid := m.GetMessageID()
  mids = append(mids, mid)