diff --git a/msg.go b/msg.go
index 758373b..6ca88ee 100644
--- a/msg.go
+++ b/msg.go
@@ -367,23 +367,45 @@ func (m *Msg) SignWithTLSCertificate(keyPairTlS *tls.Certificate) error {
 		return fmt.Errorf("failed to parse intermediate certificate: %w", err)
 	}
 
+	leafCertificate, err := getLeafCertificate(keyPairTlS)
+	if err != nil {
+		return fmt.Errorf("failed to get leaf certificate: %w", err)
+	}
+
 	switch keyPairTlS.PrivateKey.(type) {
 	case *rsa.PrivateKey:
 		if intermediateCertificate == nil {
-			return m.SignWithSMimeRSA(keyPairTlS.PrivateKey.(*rsa.PrivateKey), keyPairTlS.Leaf, nil)
+			return m.SignWithSMimeRSA(keyPairTlS.PrivateKey.(*rsa.PrivateKey), leafCertificate, nil)
 		}
-		return m.SignWithSMimeRSA(keyPairTlS.PrivateKey.(*rsa.PrivateKey), keyPairTlS.Leaf, intermediateCertificate)
+		return m.SignWithSMimeRSA(keyPairTlS.PrivateKey.(*rsa.PrivateKey), leafCertificate, intermediateCertificate)
 
 	case *ecdsa.PrivateKey:
 		if intermediateCertificate == nil {
-			return m.SignWithSMimeECDSA(keyPairTlS.PrivateKey.(*ecdsa.PrivateKey), keyPairTlS.Leaf, nil)
+			return m.SignWithSMimeECDSA(keyPairTlS.PrivateKey.(*ecdsa.PrivateKey), leafCertificate, nil)
 		}
-		return m.SignWithSMimeECDSA(keyPairTlS.PrivateKey.(*ecdsa.PrivateKey), keyPairTlS.Leaf, intermediateCertificate)
+		return m.SignWithSMimeECDSA(keyPairTlS.PrivateKey.(*ecdsa.PrivateKey), leafCertificate, intermediateCertificate)
 	default:
 		return fmt.Errorf("unsupported private key type: %T", keyPairTlS.PrivateKey)
 	}
 }
 
+// getLeafCertificate returns the leaf certificate from a tls.Certificate.
+// PLEASE NOTE: Before Go 1.23 Certificate.Leaf was left nil, and the parsed certificate was
+// discarded. This behavior can be re-enabled by setting "x509keypairleaf=0"
+// in the GODEBUG environment variable.
+func getLeafCertificate(keyPairTlS *tls.Certificate) (*x509.Certificate, error) {
+	if keyPairTlS.Leaf != nil {
+		return keyPairTlS.Leaf, nil
+	}
+
+	cert, err := x509.ParseCertificate(keyPairTlS.Certificate[0])
+	if err != nil {
+		return nil, err
+	}
+
+	return cert, nil
+}
+
 // This method allows you to specify a character set for the email message. The charset is
 // important for ensuring that the content of the message is correctly interpreted by
 // mail clients. Common charset values include UTF-8, ISO-8859-1, and others. If a charset
diff --git a/msg_test.go b/msg_test.go
index aa46f94..d984178 100644
--- a/msg_test.go
+++ b/msg_test.go
@@ -3379,6 +3379,47 @@ func TestSignWithSMime_ValidECDSAKeyPair(t *testing.T) {
 	}
 }
 
+// TestSignWithTLSCertificate tests SignWithTLSCertificate with given *tls.Certificate
+func TestSignWithTLSCertificate(t *testing.T) {
+	keyPairTLS, err := getDummyKeyPairTLS()
+	if err != nil {
+		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
+	}
+	m := NewMsg()
+	if err := m.SignWithTLSCertificate(keyPairTLS); err != nil {
+		t.Errorf("failed to set sMime. Cause: %v", err)
+	}
+	if m.sMime.privateKey.ecdsa == nil {
+		t.Errorf("SignWithTLSCertificate() - no private key is given")
+	}
+	if m.sMime.certificate == nil {
+		t.Errorf("SignWithTLSCertificate() - no certificate is given")
+	}
+}
+
+// TestSignWithTLSCertificate tests SignWithTLSCertificate with given *tls.Certificate and nil leaf certificate
+// PLEASE NOTE: Before Go 1.23 Certificate.Leaf was left nil, and the parsed certificate was
+// discarded. This behavior can be re-enabled by setting "x509keypairleaf=0"
+// in the GODEBUG environment variable.
+func TestSignWithTLSCertificate_WithKeyPairLeafNil(t *testing.T) {
+	t.Setenv("GODEBUG", "x509keypairleaf=0")
+
+	keyPairTLS, err := getDummyKeyPairTLS()
+	if err != nil {
+		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
+	}
+	m := NewMsg()
+	if err := m.SignWithTLSCertificate(keyPairTLS); err != nil {
+		t.Errorf("failed to set sMime. Cause: %v", err)
+	}
+	if m.sMime.privateKey.ecdsa == nil {
+		t.Errorf("SignWithTLSCertificate() - no private key is given")
+	}
+	if m.sMime.certificate == nil {
+		t.Errorf("SignWithTLSCertificate() - no certificate is given")
+	}
+}
+
 // TestSignWithSMime_InvalidPrivateKey tests WithSMimeSinging with given invalid private key
 func TestSignWithSMime_InvalidPrivateKey(t *testing.T) {
 	m := NewMsg()
@@ -3511,3 +3552,19 @@ func TestMsg_signMessage(t *testing.T) {
 		t.Errorf("createSignaturePart() method failed. Expected content should not be equal: %s, got: %s", body, signaturePart.GetEncoding())
 	}
 }
+
+// TestGetLeafCertificate tests the Msg.getLeafCertificate method
+func TestGetLeafCertificate(t *testing.T) {
+	keyPairTLS, err := getDummyKeyPairTLS()
+	if err != nil {
+		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
+	}
+
+	leafCertificate, err := getLeafCertificate(keyPairTLS)
+	if err != nil {
+		t.Errorf("failed to get leaf certificate. Cause: %v", err)
+	}
+	if leafCertificate == nil {
+		t.Errorf("failed to get leaf certificate")
+	}
+}
diff --git a/util_test.go b/util_test.go
index 7640f62..8a6c6bc 100644
--- a/util_test.go
+++ b/util_test.go
@@ -19,7 +19,7 @@ const (
 	keyECDSAFilePath  = "dummy-child-key-ecdsa.pem"
 )
 
-// getDummyRSACryptoMaterial loads a certificate (RSA) and the associated private key (ECDSA) form local disk for testing purposes
+// getDummyRSACryptoMaterial loads a certificate (RSA), the associated private key and certificate (RSA) is loaded from local disk for testing purposes
 func getDummyRSACryptoMaterial() (*rsa.PrivateKey, *x509.Certificate, *x509.Certificate, error) {
 	keyPair, err := tls.LoadX509KeyPair(certRSAFilePath, keyRSAFilePath)
 	if err != nil {
@@ -41,7 +41,7 @@ func getDummyRSACryptoMaterial() (*rsa.PrivateKey, *x509.Certificate, *x509.Cert
 	return privateKey, certificate, intermediateCertificate, nil
 }
 
-// getDummyECDSACryptoMaterial loads a certificate (ECDSA) and the associated private key (ECDSA) form local disk for testing purposes
+// getDummyECDSACryptoMaterial loads a certificate (ECDSA), the associated private key and certificate (ECDSA) is loaded from local disk for testing purposes
 func getDummyECDSACryptoMaterial() (*ecdsa.PrivateKey, *x509.Certificate, *x509.Certificate, error) {
 	keyPair, err := tls.LoadX509KeyPair(certECDSAFilePath, keyECDSAFilePath)
 	if err != nil {
@@ -62,3 +62,12 @@ func getDummyECDSACryptoMaterial() (*ecdsa.PrivateKey, *x509.Certificate, *x509.
 
 	return privateKey, certificate, intermediateCertificate, nil
 }
+
+// getDummyKeyPairTLS loads a certificate (ECDSA) as *tls.Certificate, the associated private key and certificate (ECDSA) is loaded from local disk for testing purposes
+func getDummyKeyPairTLS() (*tls.Certificate, error) {
+	keyPair, err := tls.LoadX509KeyPair(certECDSAFilePath, keyECDSAFilePath)
+	if err != nil {
+		return nil, err
+	}
+	return &keyPair, err
+}