diff --git a/msg.go b/msg.go
index fc0bb57..61feda1 100644
--- a/msg.go
+++ b/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.
diff --git a/msg_test.go b/msg_test.go
index f570b02..7fcbd99 100644
--- a/msg_test.go
+++ b/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)
diff --git a/random.go b/random.go
index 2478c75..e987f00 100644
--- a/random.go
+++ b/random.go
@@ -11,16 +11,17 @@ import (
 )
 
 // Range of characters for the secure string generation
-const cr = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890"
+const cr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789._-"
 
 // Bitmask sizes for the string generators (based on 93 chars total)
 //
 // These constants define bitmask-related values used for efficient random string generation.
-// The bitmask operates over 93 possible characters, and the constants help determine the
+// The bitmask operates over 66 possible characters, and the constants help determine the
 // number of bits and indices used in the process.
 const (
-	// letterIdxBits: Number of bits (7) needed to represent a letter index.
-	letterIdxBits = 7
+	// letterIdxBits: Number of bits needed to represent a letter index. We have 64 possible characters
+	// which fit into 6 bits.
+	letterIdxBits = 6
 	// letterIdxMask: Bitmask to extract letter indices (all 1-bits for letterIdxBits).
 	letterIdxMask = 1<<letterIdxBits - 1
 	// letterIdxMax: The maximum number of letter indices that fit in 63 bits.
diff --git a/random_119.go b/random_119.go
deleted file mode 100644
index 98d761c..0000000
--- a/random_119.go
+++ /dev/null
@@ -1,32 +0,0 @@
-// SPDX-FileCopyrightText: 2022-2023 The go-mail Authors
-//
-// SPDX-License-Identifier: MIT
-
-//go:build !go1.20
-// +build !go1.20
-
-package mail
-
-import (
-	"math/rand"
-	"time"
-)
-
-// randNum returns a random number with a maximum value of maxval.
-//
-// This function generates a random integer between 0 and maxval (exclusive). It seeds the
-// random number generator with the current time in nanoseconds to ensure different results
-// each time the function is called.
-//
-// Parameters:
-//   - maxval: The upper bound for the random number generation (exclusive).
-//
-// Returns:
-//   - A random integer between 0 and maxval. If maxval is less than or equal to 0, it returns 0.
-func randNum(maxval int) int {
-	if maxval <= 0 {
-		return 0
-	}
-	rand.Seed(time.Now().UnixNano())
-	return rand.Intn(maxval)
-}
diff --git a/random_121.go b/random_121.go
deleted file mode 100644
index ea1e399..0000000
--- a/random_121.go
+++ /dev/null
@@ -1,30 +0,0 @@
-// SPDX-FileCopyrightText: 2022-2023 The go-mail Authors
-//
-// SPDX-License-Identifier: MIT
-
-//go:build go1.20 && !go1.22
-// +build go1.20,!go1.22
-
-package mail
-
-import (
-	"math/rand"
-)
-
-// randNum returns a random number with a maximum value of maxval.
-//
-// This function generates a random integer between 0 and maxval (exclusive). If maxval is less
-// than or equal to 0, it returns 0. The random number generator uses the default seed provided
-// by the rand package.
-//
-// Parameters:
-//   - maxval: The upper bound for the random number generation (exclusive).
-//
-// Returns:
-//   - A random integer between 0 and maxval. If maxval is less than or equal to 0, it returns 0.
-func randNum(maxval int) int {
-	if maxval <= 0 {
-		return 0
-	}
-	return rand.Intn(maxval)
-}
diff --git a/random_122.go b/random_122.go
deleted file mode 100644
index 27ded3d..0000000
--- a/random_122.go
+++ /dev/null
@@ -1,30 +0,0 @@
-// SPDX-FileCopyrightText: 2022-2023 The go-mail Authors
-//
-// SPDX-License-Identifier: MIT
-
-//go:build go1.22
-// +build go1.22
-
-package mail
-
-import (
-	"math/rand/v2"
-)
-
-// randNum returns a random number with a maximum value of maxval.
-//
-// This function generates a random integer between 0 and maxval (exclusive). It utilizes
-// the math/rand/v2 interface for Go 1.22+ and will default to math/rand for older Go versions.
-// If maxval is less than or equal to 0, it returns 0.
-//
-// Parameters:
-//   - maxval: The upper bound for the random number generation (exclusive).
-//
-// Returns:
-//   - A random integer between 0 and maxval. If maxval is less than or equal to 0, it returns 0.
-func randNum(maxval int) int {
-	if maxval <= 0 {
-		return 0
-	}
-	return rand.IntN(maxval)
-}
diff --git a/random_test.go b/random_test.go
index e69b4e7..a608c2a 100644
--- a/random_test.go
+++ b/random_test.go
@@ -38,34 +38,12 @@ func TestRandomStringSecure(t *testing.T) {
 	}
 }
 
-// TestRandomNum tests the randomNum method
-func TestRandomNum(t *testing.T) {
-	tt := []struct {
-		testName string
-		max      int
-	}{
-		{"Max: 1", 1},
-		{"Max: 20", 20},
-		{"Max: 50", 50},
-		{"Max: 100", 100},
-		{"Max: 1000", 1000},
-		{"Max: 10000", 10000},
-		{"Max: 100000000", 100000000},
-	}
-
-	for _, tc := range tt {
-		t.Run(tc.testName, func(t *testing.T) {
-			rn := randNum(tc.max)
-			if rn > tc.max {
-				t.Errorf("random number generation failed: %d is bigger than given value %d", rn, tc.max)
-			}
-		})
-	}
-}
-
-func TestRandomNumZero(t *testing.T) {
-	rn := randNum(0)
-	if rn != 0 {
-		t.Errorf("random number generation failed: %d is not zero", rn)
+func BenchmarkGenerator_RandomStringSecure(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		_, err := randomStringSecure(22)
+		if err != nil {
+			b.Errorf("RandomStringFromCharRange() failed: %s", err)
+		}
 	}
 }