diff --git a/msg_test.go b/msg_test.go
index e133397..6e35d49 100644
--- a/msg_test.go
+++ b/msg_test.go
@@ -1894,6 +1894,23 @@ func TestMsg_hasAlt(t *testing.T) {
 	}
 }
 
+// TestMsg_hasAlt tests the hasAlt() method of the Msg with active S/MIME
+func TestMsg_hasAltWithSMime(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+	}
+	m := NewMsg()
+	m.SetBodyString(TypeTextPlain, "Plain")
+	m.AddAlternativeString(TypeTextHTML, "<b>HTML</b>")
+	if err := m.SignWithSMime(keyPair); err != nil {
+		t.Errorf("set of certificate was not successful")
+	}
+	if m.hasAlt() {
+		t.Errorf("mail has alternative parts and S/MIME is active, but hasAlt() returned true")
+	}
+}
+
 // TestMsg_hasRelated tests the hasRelated() method of the Msg
 func TestMsg_hasRelated(t *testing.T) {
 	m := NewMsg()
@@ -3233,32 +3250,33 @@ func TestNewMsgWithNoDefaultUserAgent(t *testing.T) {
 	}
 }
 
-// TestWithSMimeSinging_ValidPrivateKey tests WithSMimeSinging with given privateKey
-func TestWithSMimeSinging_ValidPrivateKey(t *testing.T) {
-	privateKey, err := getDummyPrivateKey()
+// TestSignWithSMime_ValidKeyPair tests WithSMimeSinging with given key pair
+func TestSignWithSMime_ValidKeyPair(t *testing.T) {
+	keyPair, err := getDummyCertificate()
 	if err != nil {
-		t.Errorf("failed to load dummy private key: %s", err)
+		t.Errorf("failed to load dummy certificate. Cause: %v", err)
 	}
-	certificate, err := getDummyCertificate(privateKey)
-	if err != nil {
-		t.Errorf("failed to load dummy certificate: %s", err)
-	}
-
 	m := NewMsg()
-	if err := m.SignWithSMime(privateKey, certificate); err != nil {
+	if err := m.SignWithSMime(keyPair); err != nil {
 		t.Errorf("failed to set sMime. Cause: %v", err)
 	}
-	if m.sMime.privateKey != privateKey {
-		t.Errorf("WithSMimeSinging. Expected %v, got: %v", privateKey, m.sMime.privateKey)
+	if m.sMime.privateKey == nil {
+		t.Errorf("WithSMimeSinging() - no private key is given")
+	}
+	if m.sMime.certificate == nil {
+		t.Errorf("WithSMimeSinging() - no certificate is given")
+	}
+	if len(m.sMime.parentCertificates) != len(keyPair.Certificate[:1]) {
+		t.Errorf("WithSMimeSinging() - no certificate is given")
 	}
 }
 
-// TestWithSMimeSinging_InvalidPrivateKey tests WithSMimeSinging with given invalid privateKey
-func TestWithSMimeSinging_InvalidPrivateKey(t *testing.T) {
+// TestSignWithSMime_InvalidKeyPair tests WithSMimeSinging with given invalid key pair
+func TestSignWithSMime_InvalidKeyPair(t *testing.T) {
 	m := NewMsg()
 
-	err := m.SignWithSMime(nil, nil)
-	if !errors.Is(err, ErrInvalidPrivateKey) {
+	err := m.SignWithSMime(nil)
+	if !errors.Is(err, ErrInvalidKeyPair) {
 		t.Errorf("failed to check sMimeAuthConfig values correctly: %s", err)
 	}
 }
@@ -3290,3 +3308,83 @@ func FuzzMsg_From(f *testing.F) {
 		m.Reset()
 	})
 }
+
+// TestMsg_createSignaturePart tests the Msg.createSignaturePart method
+func TestMsg_createSignaturePart(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+	}
+	m := NewMsg()
+	if err := m.SignWithSMime(keyPair); err != nil {
+		t.Errorf("set of certificate was not successful")
+	}
+	body := []byte("This is the body")
+	part, err := m.createSignaturePart(EncodingQP, TypeTextPlain, CharsetUTF7, body)
+	if err != nil {
+		t.Errorf("createSignaturePart() method failed.")
+	}
+
+	if part.GetEncoding() != EncodingB64 {
+		t.Errorf("createSignaturePart() method failed. Expected encoding: %s, got: %s", EncodingB64, part.GetEncoding())
+	}
+	if part.GetContentType() != typeSMimeSigned {
+		t.Errorf("createSignaturePart() method failed. Expected content type: %s, got: %s", typeSMimeSigned, part.GetContentType())
+	}
+	if part.GetCharset() != CharsetUTF8 {
+		t.Errorf("createSignaturePart() method failed. Expected charset: %s, got: %s", CharsetUTF8, part.GetCharset())
+	}
+	if content, err := part.GetContent(); err != nil || len(content) == len(body) {
+		t.Errorf("createSignaturePart() method failed. Expected content should not be equal: %s, got: %s", body, part.GetEncoding())
+	}
+}
+
+// TestMsg_signMessage tests the Msg.signMessage method
+func TestMsg_signMessage(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+	}
+	m := NewMsg()
+	body := []byte("This is the body")
+	if err := m.SignWithSMime(keyPair); err != nil {
+		t.Errorf("set of certificate was not successful")
+	}
+	msg, err := m.signMessage(m)
+	if err != nil {
+		t.Errorf("createSignaturePart() method failed.")
+	}
+
+	parts := msg.GetParts()
+	if len(parts) != 2 {
+		t.Errorf("createSignaturePart() method failed. Expected 2 parts, got: %d", len(parts))
+	}
+
+	signedPart := parts[0]
+	if signedPart.GetEncoding() != EncodingQP {
+		t.Errorf("createSignaturePart() method failed. Expected encoding: %s, got: %s", EncodingB64, signedPart.GetEncoding())
+	}
+	if signedPart.GetContentType() != TypeTextPlain {
+		t.Errorf("createSignaturePart() method failed. Expected content type: %s, got: %s", typeSMimeSigned, signedPart.GetContentType())
+	}
+	if signedPart.GetCharset() != CharsetUTF8 {
+		t.Errorf("createSignaturePart() method failed. Expected charset: %s, got: %s", CharsetUTF8, signedPart.GetCharset())
+	}
+	if content, err := signedPart.GetContent(); err != nil || len(content) != len(body) {
+		t.Errorf("createSignaturePart() method failed. Expected content should be equal: %s, got: %s", body, content)
+	}
+
+	signaturePart := parts[1]
+	if signaturePart.GetEncoding() != EncodingB64 {
+		t.Errorf("createSignaturePart() method failed. Expected encoding: %s, got: %s", EncodingB64, signaturePart.GetEncoding())
+	}
+	if signaturePart.GetContentType() != typeSMimeSigned {
+		t.Errorf("createSignaturePart() method failed. Expected content type: %s, got: %s", typeSMimeSigned, signaturePart.GetContentType())
+	}
+	if signaturePart.GetCharset() != CharsetUTF8 {
+		t.Errorf("createSignaturePart() method failed. Expected charset: %s, got: %s", CharsetUTF8, signaturePart.GetCharset())
+	}
+	if content, err := signaturePart.GetContent(); err != nil || len(content) == len(body) {
+		t.Errorf("createSignaturePart() method failed. Expected content should not be equal: %s, got: %s", body, signaturePart.GetEncoding())
+	}
+}
diff --git a/msgwriter_test.go b/msgwriter_test.go
index a41e5d3..ca1a982 100644
--- a/msgwriter_test.go
+++ b/msgwriter_test.go
@@ -154,3 +154,26 @@ func TestMsgWriter_writeMsg_PGP(t *testing.T) {
 		t.Errorf("writeMsg failed. Expected PGP encoding header but didn't find it in message output")
 	}
 }
+
+// TestMsgWriter_writeMsg_SMime tests the writeMsg method of the msgWriter with S/MIME types set
+func TestMsgWriter_writeMsg_SMime(t *testing.T) {
+	keyPair, err := getDummyCertificate()
+	if err != nil {
+		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+	}
+	m := NewMsg()
+	if err := m.SignWithSMime(keyPair); err != nil {
+		t.Errorf("set of certificate was not successful")
+	}
+	_ = m.From(`"Toni Tester" <test@example.com>`)
+	_ = m.To(`"Toni Receiver" <receiver@example.com>`)
+	m.Subject("This is a subject")
+	m.SetBodyString(TypeTextPlain, "This is the body")
+	buf := bytes.Buffer{}
+	mw := &msgWriter{writer: &buf, charset: CharsetUTF8, encoder: mime.QEncoding}
+	mw.writeMsg(m)
+	ms := buf.String()
+	if !strings.Contains(ms, `multipart/signed; protocol="application/pkcs7-signature"; micalg=sha256;`) {
+		t.Errorf("writeMsg failed. Expected PGP encoding header but didn't find it in message output")
+	}
+}
diff --git a/util_test.go b/util_test.go
index efc088c..8f235ed 100644
--- a/util_test.go
+++ b/util_test.go
@@ -1,41 +1,19 @@
 package mail
 
 import (
-	"crypto/rand"
-	"crypto/rsa"
-	"crypto/x509"
-	"crypto/x509/pkix"
-	"math/big"
-	"time"
+	"crypto/tls"
 )
 
-func getDummyPrivateKey() (*rsa.PrivateKey, error) {
-	privateKey, err := rsa.GenerateKey(rand.Reader, 2048)
+const (
+	certFilePath = "dummy-cert.pem"
+	keyFilePath  = "dummy=key.pem"
+)
+
+func getDummyCertificate() (*tls.Certificate, error) {
+	keyPair, err := tls.LoadX509KeyPair(certFilePath, keyFilePath)
 	if err != nil {
 		return nil, err
 	}
-	return privateKey, nil
-}
-
-func getDummyCertificate(privateKey *rsa.PrivateKey) (*x509.Certificate, error) {
-	template := &x509.Certificate{
-		SerialNumber: big.NewInt(1234),
-		Subject:      pkix.Name{Organization: []string{"My Organization"}},
-		NotBefore:    time.Now(),
-		NotAfter:     time.Now().AddDate(1, 0, 0),
-		KeyUsage:     x509.KeyUsageDigitalSignature,
-		ExtKeyUsage:  []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
-	}
-
-	certDER, err := x509.CreateCertificate(rand.Reader, template, template, &privateKey.PublicKey, privateKey)
-	if err != nil {
-		return nil, err
-	}
-
-	cert, err := x509.ParseCertificate(certDER)
-	if err != nil {
-		return nil, err
-	}
-
-	return cert, nil
+
+	return &keyPair, nil
 }