diff --git a/msg.go b/msg.go
index 455ba3b..60f758f 100644
--- a/msg.go
+++ b/msg.go
@@ -7,7 +7,8 @@ package mail
 import (
 	"bytes"
 	"context"
-	"crypto/tls"
+	"crypto/rsa"
+	"crypto/x509"
 	"embed"
 	"errors"
 	"fmt"
@@ -338,8 +339,8 @@ func WithNoDefaultUserAgent() MsgOption {
 }
 
 // SignWithSMime configures the Msg to be signed with S/MIME
-func (m *Msg) SignWithSMime(keyPair *tls.Certificate) error {
-	sMime, err := newSMime(keyPair)
+func (m *Msg) SignWithSMime(privateKey *rsa.PrivateKey, certificate *x509.Certificate, intermediateCertificate *x509.Certificate) error {
+	sMime, err := newSMime(privateKey, certificate, intermediateCertificate)
 	if err != nil {
 		return err
 	}
diff --git a/msg_test.go b/msg_test.go
index ffd1357..7842cda 100644
--- a/msg_test.go
+++ b/msg_test.go
@@ -1909,15 +1909,15 @@ 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()
+	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
 	if err != nil {
-		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+		t.Errorf("failed to laod dummy crypto material. 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 err := m.SignWithSMime(privateKey, certificate, intermediateCertificate); err != nil {
+		t.Errorf("failed to init smime configuration")
 	}
 	if m.hasAlt() {
 		t.Errorf("mail has alternative parts and S/MIME is active, but hasAlt() returned true")
@@ -1926,13 +1926,13 @@ func TestMsg_hasAltWithSMime(t *testing.T) {
 
 // TestMsg_hasSMime tests the hasSMime() method of the Msg
 func TestMsg_hasSMime(t *testing.T) {
-	keyPair, err := getDummyCertificate()
+	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
 	if err != nil {
-		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
 	}
 	m := NewMsg()
-	if err := m.SignWithSMime(keyPair); err != nil {
-		t.Errorf("set of certificate was not successful")
+	if err := m.SignWithSMime(privateKey, certificate, intermediateCertificate); err != nil {
+		t.Errorf("failed to init smime configuration")
 	}
 	m.SetBodyString(TypeTextPlain, "Plain")
 	if !m.hasSMime() {
@@ -2009,16 +2009,16 @@ func TestMsg_WriteToSkipMiddleware(t *testing.T) {
 
 // TestMsg_WriteToWithSMIME tests the WriteTo() method of the Msg
 func TestMsg_WriteToWithSMIME(t *testing.T) {
-	keyPair, err := getDummyCertificate()
+	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
 	if err != nil {
-		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
 	}
 
 	m := NewMsg()
 	m.Subject("This is a test")
 	m.SetBodyString(TypeTextPlain, "Plain")
-	if err := m.SignWithSMime(keyPair); err != nil {
-		t.Errorf("set of certificate was not successful")
+	if err := m.SignWithSMime(privateKey, certificate, intermediateCertificate); err != nil {
+		t.Errorf("failed to init smime configuration")
 	}
 
 	wbuf := bytes.Buffer{}
@@ -3345,12 +3345,12 @@ func TestNewMsgWithNoDefaultUserAgent(t *testing.T) {
 
 // TestSignWithSMime_ValidKeyPair tests WithSMimeSinging with given key pair
 func TestSignWithSMime_ValidKeyPair(t *testing.T) {
-	keyPair, err := getDummyCertificate()
+	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
 	if err != nil {
-		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
 	}
 	m := NewMsg()
-	if err := m.SignWithSMime(keyPair); err != nil {
+	if err := m.SignWithSMime(privateKey, certificate, intermediateCertificate); err != nil {
 		t.Errorf("failed to set sMime. Cause: %v", err)
 	}
 	if m.sMime.privateKey == nil {
@@ -3361,13 +3361,41 @@ func TestSignWithSMime_ValidKeyPair(t *testing.T) {
 	}
 }
 
-// TestSignWithSMime_InvalidKeyPair tests WithSMimeSinging with given invalid key pair
-func TestSignWithSMime_InvalidKeyPair(t *testing.T) {
+// TestSignWithSMime_InvalidPrivateKey tests WithSMimeSinging with given invalid private key
+func TestSignWithSMime_InvalidPrivateKey(t *testing.T) {
 	m := NewMsg()
 
-	err := m.SignWithSMime(nil)
-	if !errors.Is(err, ErrInvalidKeyPair) {
-		t.Errorf("failed to check sMimeAuthConfig values correctly: %s", err)
+	err := m.SignWithSMime(nil, nil, nil)
+	if !errors.Is(err, ErrInvalidPrivateKey) {
+		t.Errorf("failed to pre-check SignWithSMime method values correctly: %s", err)
+	}
+}
+
+// TestSignWithSMime_InvalidCertificate tests WithSMimeSinging with given invalid certificate
+func TestSignWithSMime_InvalidCertificate(t *testing.T) {
+	privateKey, _, _, err := getDummyCryptoMaterial()
+	if err != nil {
+		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
+	}
+	m := NewMsg()
+
+	err = m.SignWithSMime(privateKey, nil, nil)
+	if !errors.Is(err, ErrInvalidCertificate) {
+		t.Errorf("failed to pre-check SignWithSMime method values correctly: %s", err)
+	}
+}
+
+// TestSignWithSMime_InvalidIntermediateCertificate tests WithSMimeSinging with given invalid intermediate certificate
+func TestSignWithSMime_InvalidIntermediateCertificate(t *testing.T) {
+	privateKey, certificate, _, err := getDummyCryptoMaterial()
+	if err != nil {
+		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
+	}
+	m := NewMsg()
+
+	err = m.SignWithSMime(privateKey, certificate, nil)
+	if !errors.Is(err, ErrInvalidIntermediateCertificate) {
+		t.Errorf("failed to pre-check SignWithSMime method values correctly: %s", err)
 	}
 }
 
@@ -3401,13 +3429,13 @@ func FuzzMsg_From(f *testing.F) {
 
 // TestMsg_createSignaturePart tests the Msg.createSignaturePart method
 func TestMsg_createSignaturePart(t *testing.T) {
-	keyPair, err := getDummyCertificate()
+	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
 	if err != nil {
-		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
 	}
 	m := NewMsg()
-	if err := m.SignWithSMime(keyPair); err != nil {
-		t.Errorf("set of certificate was not successful")
+	if err := m.SignWithSMime(privateKey, certificate, intermediateCertificate); err != nil {
+		t.Errorf("failed to init smime configuration")
 	}
 	body := []byte("This is the body")
 	part, err := m.createSignaturePart(EncodingQP, TypeTextPlain, CharsetUTF7, body)
@@ -3431,15 +3459,15 @@ func TestMsg_createSignaturePart(t *testing.T) {
 
 // TestMsg_signMessage tests the Msg.signMessage method
 func TestMsg_signMessage(t *testing.T) {
-	keyPair, err := getDummyCertificate()
+	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
 	if err != nil {
-		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
 	}
 	body := []byte("This is the body")
 	m := NewMsg()
 	m.SetBodyString(TypeTextPlain, string(body))
-	if err := m.SignWithSMime(keyPair); err != nil {
-		t.Errorf("set of certificate was not successful")
+	if err := m.SignWithSMime(privateKey, certificate, intermediateCertificate); err != nil {
+		t.Errorf("failed to init smime configuration")
 	}
 	msg, err := m.signMessage(m)
 	if err != nil {
diff --git a/msgwriter_test.go b/msgwriter_test.go
index 15e751f..6e36c4b 100644
--- a/msgwriter_test.go
+++ b/msgwriter_test.go
@@ -157,14 +157,14 @@ func TestMsgWriter_writeMsg_PGP(t *testing.T) {
 
 // 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()
+	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
 	if err != nil {
-		t.Errorf("failed to load dummy certificate. Cause: %v", err)
+		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
 	}
 
 	m := NewMsg()
-	if err := m.SignWithSMime(keyPair); err != nil {
-		t.Errorf("set of certificate was not successful")
+	if err := m.SignWithSMime(privateKey, certificate, intermediateCertificate); err != nil {
+		t.Errorf("failed to init smime configuration")
 	}
 	_ = m.From(`"Toni Tester" <test@example.com>`)
 	_ = m.To(`"Toni Receiver" <receiver@example.com>`)
diff --git a/smime.go b/smime.go
index ae6e4f2..37fb536 100644
--- a/smime.go
+++ b/smime.go
@@ -3,7 +3,6 @@ package mail
 import (
 	"bytes"
 	"crypto/rsa"
-	"crypto/tls"
 	"crypto/x509"
 	"encoding/pem"
 	"errors"
@@ -13,11 +12,14 @@ import (
 )
 
 var (
-	// ErrInvalidKeyPair should be used if key pair is invalid
-	ErrInvalidKeyPair = errors.New("invalid key pair")
+	// ErrInvalidPrivateKey should be used if private key is invalid
+	ErrInvalidPrivateKey = errors.New("invalid private key")
 
-	// ErrInvalidParentCertificates should be used if one of the parent certificates is invalid
-	ErrInvalidParentCertificates = errors.New("invalid parent certificates")
+	// ErrInvalidCertificate should be used if the certificate is invalid
+	ErrInvalidCertificate = errors.New("invalid certificate")
+
+	// ErrInvalidIntermediateCertificate should be used if the intermediate certificate is invalid
+	ErrInvalidIntermediateCertificate = errors.New("invalid intermediate certificate")
 
 	// ErrCouldNotInitialize should be used if the signed data could not initialize
 	ErrCouldNotInitialize = errors.New("could not initialize signed data")
@@ -34,30 +36,32 @@ var (
 
 // SMime is used to sign messages with S/MIME
 type SMime struct {
-	privateKey         *rsa.PrivateKey
-	parentCertificates []*x509.Certificate
-	certificate        *x509.Certificate
+	privateKey              *rsa.PrivateKey
+	certificate             *x509.Certificate
+	intermediateCertificate *x509.Certificate
 }
 
-// NewSMime construct a new instance of SMime with a provided *tls.Certificate
-func newSMime(keyPair *tls.Certificate) (*SMime, error) {
-	if keyPair == nil {
-		return nil, ErrInvalidKeyPair
+// NewSMime construct a new instance of SMime with provided parameters
+// privateKey as *rsa.PrivateKey
+// certificate as *x509.Certificate
+// intermediateCertificate as *x509.Certificate
+func newSMime(privateKey *rsa.PrivateKey, certificate *x509.Certificate, intermediateCertificate *x509.Certificate) (*SMime, error) {
+	if privateKey == nil {
+		return nil, ErrInvalidPrivateKey
 	}
 
-	parentCertificates := make([]*x509.Certificate, 0)
-	for _, cert := range keyPair.Certificate[1:] {
-		c, err := x509.ParseCertificate(cert)
-		if err != nil {
-			return nil, ErrInvalidParentCertificates
-		}
-		parentCertificates = append(parentCertificates, c)
+	if certificate == nil {
+		return nil, ErrInvalidCertificate
+	}
+
+	if intermediateCertificate == nil {
+		return nil, ErrInvalidIntermediateCertificate
 	}
 
 	return &SMime{
-		privateKey:         keyPair.PrivateKey.(*rsa.PrivateKey),
-		certificate:        keyPair.Leaf,
-		parentCertificates: parentCertificates,
+		privateKey:              privateKey,
+		certificate:             certificate,
+		intermediateCertificate: intermediateCertificate,
 	}, nil
 }
 
@@ -72,7 +76,7 @@ func (sm *SMime) signMessage(message string) (*string, error) {
 		return nil, ErrCouldNotInitialize
 	}
 
-	if err = signedData.AddSignerChain(sm.certificate, sm.privateKey, sm.parentCertificates, pkcs7.SignerInfoConfig{}); err != nil {
+	if err = signedData.AddSignerChain(sm.certificate, sm.privateKey, []*x509.Certificate{sm.intermediateCertificate}, pkcs7.SignerInfoConfig{}); err != nil {
 		return nil, ErrCouldNotAddSigner
 	}
 
diff --git a/smime_test.go b/smime_test.go
index cfd8518..bf4e2d0 100644
--- a/smime_test.go
+++ b/smime_test.go
@@ -10,32 +10,35 @@ import (
 
 // TestNewSMime tests the newSMime method
 func TestNewSMime(t *testing.T) {
-	keyPair, err := getDummyCertificate()
+	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
 	if err != nil {
-		t.Errorf("Error getting dummy certificate: %s", err)
+		t.Errorf("Error getting dummy crypto material: %s", err)
 	}
 
-	sMime, err := newSMime(keyPair)
+	sMime, err := newSMime(privateKey, certificate, intermediateCertificate)
 	if err != nil {
 		t.Errorf("Error creating new SMime from keyPair: %s", err)
 	}
 
-	if sMime.privateKey != keyPair.PrivateKey {
+	if sMime.privateKey != privateKey {
 		t.Errorf("NewSMime() did not return the same private key")
 	}
-	if sMime.certificate != keyPair.Leaf {
-		t.Errorf("NewSMime() did not return the same leaf certificate")
+	if sMime.certificate != certificate {
+		t.Errorf("NewSMime() did not return the same certificate")
+	}
+	if sMime.intermediateCertificate != intermediateCertificate {
+		t.Errorf("NewSMime() did not return the same intermedidate certificate")
 	}
 }
 
 // TestSign tests the sign method
 func TestSign(t *testing.T) {
-	keyPair, err := getDummyCertificate()
+	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
 	if err != nil {
-		t.Errorf("Error getting dummy certificate: %s", err)
+		t.Errorf("Error getting dummy crypto material: %s", err)
 	}
 
-	sMime, err := newSMime(keyPair)
+	sMime, err := newSMime(privateKey, certificate, intermediateCertificate)
 	if err != nil {
 		t.Errorf("Error creating new SMime from keyPair: %s", err)
 	}
@@ -53,12 +56,12 @@ func TestSign(t *testing.T) {
 
 // TestPrepareMessage tests the createMessage method
 func TestPrepareMessage(t *testing.T) {
-	keyPair, err := getDummyCertificate()
+	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
 	if err != nil {
-		t.Errorf("Error getting dummy certificate: %s", err)
+		t.Errorf("Error getting dummy crypto material: %s", err)
 	}
 
-	sMime, err := newSMime(keyPair)
+	sMime, err := newSMime(privateKey, certificate, intermediateCertificate)
 	if err != nil {
 		t.Errorf("Error creating new SMime from keyPair: %s", err)
 	}
diff --git a/util_test.go b/util_test.go
index 7c54b7d..33615d8 100644
--- a/util_test.go
+++ b/util_test.go
@@ -1,6 +1,7 @@
 package mail
 
 import (
+	"crypto/rsa"
 	"crypto/tls"
 	"crypto/x509"
 )
@@ -10,14 +11,24 @@ const (
 	keyFilePath  = "dummy-child-key.pem"
 )
 
-// getDummyCertificate loads a certificate and a private key form local disk for testing purposes
-func getDummyCertificate() (*tls.Certificate, error) {
+// getDummyCryptoMaterial loads a certificate and a private key form local disk for testing purposes
+func getDummyCryptoMaterial() (*rsa.PrivateKey, *x509.Certificate, *x509.Certificate, error) {
 	keyPair, err := tls.LoadX509KeyPair(certFilePath, keyFilePath)
 	if err != nil {
-		return nil, err
+		return nil, nil, nil, err
 	}
 
-	keyPair.Leaf, err = x509.ParseCertificate(keyPair.Certificate[0])
+	privateKey := keyPair.PrivateKey.(*rsa.PrivateKey)
 
-	return &keyPair, nil
+	certificate, err := x509.ParseCertificate(keyPair.Certificate[0])
+	if err != nil {
+		return nil, nil, nil, err
+	}
+
+	intermediateCertificate, err := x509.ParseCertificate(keyPair.Certificate[1])
+	if err != nil {
+		return nil, nil, nil, err
+	}
+
+	return privateKey, certificate, intermediateCertificate, nil
 }