diff --git a/msg.go b/msg.go
index 60f758f..7f5bbd9 100644
--- a/msg.go
+++ b/msg.go
@@ -7,7 +7,9 @@ package mail
 import (
 	"bytes"
 	"context"
+	"crypto/ecdsa"
 	"crypto/rsa"
+	"crypto/tls"
 	"crypto/x509"
 	"embed"
 	"errors"
@@ -338,9 +340,9 @@ func WithNoDefaultUserAgent() MsgOption {
 	}
 }
 
-// SignWithSMime configures the Msg to be signed with S/MIME
-func (m *Msg) SignWithSMime(privateKey *rsa.PrivateKey, certificate *x509.Certificate, intermediateCertificate *x509.Certificate) error {
-	sMime, err := newSMime(privateKey, certificate, intermediateCertificate)
+// SignWithSMimeRSA configures the Msg to be signed with S/MIME
+func (m *Msg) SignWithSMimeRSA(privateKey *rsa.PrivateKey, certificate *x509.Certificate, intermediateCertificate *x509.Certificate) error {
+	sMime, err := newSMimeWithRSA(privateKey, certificate, intermediateCertificate)
 	if err != nil {
 		return err
 	}
@@ -348,6 +350,40 @@ func (m *Msg) SignWithSMime(privateKey *rsa.PrivateKey, certificate *x509.Certif
 	return nil
 }
 
+// SignWithSMimeECDSA configures the Msg to be signed with S/MIME
+func (m *Msg) SignWithSMimeECDSA(privateKey *ecdsa.PrivateKey, certificate *x509.Certificate, intermediateCertificate *x509.Certificate) error {
+	sMime, err := newSMimeWithECDSA(privateKey, certificate, intermediateCertificate)
+	if err != nil {
+		return err
+	}
+	m.sMime = sMime
+	return nil
+}
+
+// SignWithTLSCertificate signs the Msg with the provided *tls.certificate.
+func (m *Msg) SignWithTLSCertificate(keyPairTlS *tls.Certificate) error {
+	intermediateCertificate, err := x509.ParseCertificate(keyPairTlS.Certificate[1])
+	if err != nil {
+		return fmt.Errorf("failed to parse intermediate 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), keyPairTlS.Leaf, intermediateCertificate)
+
+	case *ecdsa.PrivateKey:
+		if intermediateCertificate == nil {
+			return m.SignWithSMimeECDSA(keyPairTlS.PrivateKey.(*ecdsa.PrivateKey), keyPairTlS.Leaf, nil)
+		}
+		return m.SignWithSMimeECDSA(keyPairTlS.PrivateKey.(*ecdsa.PrivateKey), keyPairTlS.Leaf, intermediateCertificate)
+	default:
+		return fmt.Errorf("unsupported private key type: %T", keyPairTlS.PrivateKey)
+	}
+}
+
 // 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 7842cda..2aeab91 100644
--- a/msg_test.go
+++ b/msg_test.go
@@ -1909,14 +1909,14 @@ 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) {
-	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
+	privateKey, certificate, intermediateCertificate, err := getDummyRSACryptoMaterial()
 	if err != nil {
 		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(privateKey, certificate, intermediateCertificate); err != nil {
+	if err := m.SignWithSMimeRSA(privateKey, certificate, intermediateCertificate); err != nil {
 		t.Errorf("failed to init smime configuration")
 	}
 	if m.hasAlt() {
@@ -1926,12 +1926,12 @@ func TestMsg_hasAltWithSMime(t *testing.T) {
 
 // TestMsg_hasSMime tests the hasSMime() method of the Msg
 func TestMsg_hasSMime(t *testing.T) {
-	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
+	privateKey, certificate, intermediateCertificate, err := getDummyRSACryptoMaterial()
 	if err != nil {
 		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
 	}
 	m := NewMsg()
-	if err := m.SignWithSMime(privateKey, certificate, intermediateCertificate); err != nil {
+	if err := m.SignWithSMimeRSA(privateKey, certificate, intermediateCertificate); err != nil {
 		t.Errorf("failed to init smime configuration")
 	}
 	m.SetBodyString(TypeTextPlain, "Plain")
@@ -2009,7 +2009,7 @@ func TestMsg_WriteToSkipMiddleware(t *testing.T) {
 
 // TestMsg_WriteToWithSMIME tests the WriteTo() method of the Msg
 func TestMsg_WriteToWithSMIME(t *testing.T) {
-	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
+	privateKey, certificate, intermediateCertificate, err := getDummyRSACryptoMaterial()
 	if err != nil {
 		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
 	}
@@ -2017,7 +2017,7 @@ func TestMsg_WriteToWithSMIME(t *testing.T) {
 	m := NewMsg()
 	m.Subject("This is a test")
 	m.SetBodyString(TypeTextPlain, "Plain")
-	if err := m.SignWithSMime(privateKey, certificate, intermediateCertificate); err != nil {
+	if err := m.SignWithSMimeRSA(privateKey, certificate, intermediateCertificate); err != nil {
 		t.Errorf("failed to init smime configuration")
 	}
 
@@ -3343,17 +3343,35 @@ func TestNewMsgWithNoDefaultUserAgent(t *testing.T) {
 	}
 }
 
-// TestSignWithSMime_ValidKeyPair tests WithSMimeSinging with given key pair
-func TestSignWithSMime_ValidKeyPair(t *testing.T) {
-	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
+// TestSignWithSMime_ValidRSAKeyPair tests WithSMimeSinging with given rsa key pair
+func TestSignWithSMime_ValidRSAKeyPair(t *testing.T) {
+	privateKey, certificate, intermediateCertificate, err := getDummyRSACryptoMaterial()
 	if err != nil {
 		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
 	}
 	m := NewMsg()
-	if err := m.SignWithSMime(privateKey, certificate, intermediateCertificate); err != nil {
+	if err := m.SignWithSMimeRSA(privateKey, certificate, intermediateCertificate); err != nil {
 		t.Errorf("failed to set sMime. Cause: %v", err)
 	}
-	if m.sMime.privateKey == nil {
+	if m.sMime.privateKey.rsa == nil {
+		t.Errorf("WithSMimeSinging() - no private key is given")
+	}
+	if m.sMime.certificate == nil {
+		t.Errorf("WithSMimeSinging() - no certificate is given")
+	}
+}
+
+// TestSignWithSMime_ValidRSAKeyPair tests WithSMimeSinging with given ecdsa key pair
+func TestSignWithSMime_ValidECDSAKeyPair(t *testing.T) {
+	privateKey, certificate, intermediateCertificate, err := getDummyECDSACryptoMaterial()
+	if err != nil {
+		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
+	}
+	m := NewMsg()
+	if err := m.SignWithSMimeECDSA(privateKey, certificate, intermediateCertificate); err != nil {
+		t.Errorf("failed to set sMime. Cause: %v", err)
+	}
+	if m.sMime.privateKey.rsa == nil {
 		t.Errorf("WithSMimeSinging() - no private key is given")
 	}
 	if m.sMime.certificate == nil {
@@ -3365,7 +3383,7 @@ func TestSignWithSMime_ValidKeyPair(t *testing.T) {
 func TestSignWithSMime_InvalidPrivateKey(t *testing.T) {
 	m := NewMsg()
 
-	err := m.SignWithSMime(nil, nil, nil)
+	err := m.SignWithSMimeRSA(nil, nil, nil)
 	if !errors.Is(err, ErrInvalidPrivateKey) {
 		t.Errorf("failed to pre-check SignWithSMime method values correctly: %s", err)
 	}
@@ -3373,32 +3391,18 @@ func TestSignWithSMime_InvalidPrivateKey(t *testing.T) {
 
 // TestSignWithSMime_InvalidCertificate tests WithSMimeSinging with given invalid certificate
 func TestSignWithSMime_InvalidCertificate(t *testing.T) {
-	privateKey, _, _, err := getDummyCryptoMaterial()
+	privateKey, _, _, err := getDummyRSACryptoMaterial()
 	if err != nil {
 		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
 	}
 	m := NewMsg()
 
-	err = m.SignWithSMime(privateKey, nil, nil)
+	err = m.SignWithSMimeRSA(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)
-	}
-}
-
 // Fuzzing tests
 func FuzzMsg_Subject(f *testing.F) {
 	f.Add("Testsubject")
@@ -3429,12 +3433,12 @@ func FuzzMsg_From(f *testing.F) {
 
 // TestMsg_createSignaturePart tests the Msg.createSignaturePart method
 func TestMsg_createSignaturePart(t *testing.T) {
-	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
+	privateKey, certificate, intermediateCertificate, err := getDummyRSACryptoMaterial()
 	if err != nil {
 		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
 	}
 	m := NewMsg()
-	if err := m.SignWithSMime(privateKey, certificate, intermediateCertificate); err != nil {
+	if err := m.SignWithSMimeRSA(privateKey, certificate, intermediateCertificate); err != nil {
 		t.Errorf("failed to init smime configuration")
 	}
 	body := []byte("This is the body")
@@ -3459,14 +3463,14 @@ func TestMsg_createSignaturePart(t *testing.T) {
 
 // TestMsg_signMessage tests the Msg.signMessage method
 func TestMsg_signMessage(t *testing.T) {
-	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
+	privateKey, certificate, intermediateCertificate, err := getDummyRSACryptoMaterial()
 	if err != nil {
 		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(privateKey, certificate, intermediateCertificate); err != nil {
+	if err := m.SignWithSMimeRSA(privateKey, certificate, intermediateCertificate); err != nil {
 		t.Errorf("failed to init smime configuration")
 	}
 	msg, err := m.signMessage(m)
diff --git a/msgwriter_test.go b/msgwriter_test.go
index 6e36c4b..0379cd7 100644
--- a/msgwriter_test.go
+++ b/msgwriter_test.go
@@ -157,13 +157,13 @@ 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) {
-	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
+	privateKey, certificate, intermediateCertificate, err := getDummyRSACryptoMaterial()
 	if err != nil {
 		t.Errorf("failed to laod dummy crypto material. Cause: %v", err)
 	}
 
 	m := NewMsg()
-	if err := m.SignWithSMime(privateKey, certificate, intermediateCertificate); err != nil {
+	if err := m.SignWithSMimeRSA(privateKey, certificate, intermediateCertificate); err != nil {
 		t.Errorf("failed to init smime configuration")
 	}
 	_ = m.From(`"Toni Tester" <test@example.com>`)
diff --git a/pkcs7.go b/pkcs7.go
index 6522bb6..f16b54f 100644
--- a/pkcs7.go
+++ b/pkcs7.go
@@ -25,7 +25,7 @@ import (
 type PKCS7 struct {
 	Content      []byte
 	Certificates []*x509.Certificate
-	CRLs         []pkix.CertificateList
+	CRLs         []x509.RevocationList
 	Signers      []signerInfo
 	raw          interface{}
 }
@@ -47,9 +47,9 @@ type signedData struct {
 	Version                    int                        `asn1:"default:1"`
 	DigestAlgorithmIdentifiers []pkix.AlgorithmIdentifier `asn1:"set"`
 	ContentInfo                contentInfo
-	Certificates               rawCertificates        `asn1:"optional,tag:0"`
-	CRLs                       []pkix.CertificateList `asn1:"optional,tag:1"`
-	SignerInfos                []signerInfo           `asn1:"set"`
+	Certificates               rawCertificates       `asn1:"optional,tag:0"`
+	CRLs                       []x509.RevocationList `asn1:"optional,tag:1"`
+	SignerInfos                []signerInfo          `asn1:"set"`
 }
 
 type rawCertificates struct {
@@ -110,7 +110,9 @@ func marshalAttributes(attrs []attribute) ([]byte, error) {
 
 	// Remove the leading sequence octets
 	var raw asn1.RawValue
-	asn1.Unmarshal(encodedAttributes, &raw)
+	if _, err := asn1.Unmarshal(encodedAttributes, &raw); err != nil {
+		return nil, err
+	}
 	return raw.Bytes, nil
 }
 
@@ -377,7 +379,7 @@ func marshalCertificates(certs []*x509.Certificate) rawCertificates {
 // RawContent, we have to encode it into the RawContent. If its missing,
 // then `asn1.Marshal()` will strip out the certificate wrapper instead.
 func marshalCertificateBytes(certs []byte) (rawCertificates, error) {
-	var val = asn1.RawValue{Bytes: certs, Class: 2, Tag: 0, IsCompound: true}
+	val := asn1.RawValue{Bytes: certs, Class: 2, Tag: 0, IsCompound: true}
 	b, err := asn1.Marshal(val)
 	if err != nil {
 		return rawCertificates{}, err
diff --git a/pkcs7_test.go b/pkcs7_test.go
index fa79faf..7a65644 100644
--- a/pkcs7_test.go
+++ b/pkcs7_test.go
@@ -12,7 +12,6 @@ import (
 	"crypto/x509/pkix"
 	"encoding/pem"
 	"fmt"
-	"io/ioutil"
 	"math/big"
 	"os"
 	"os/exec"
@@ -75,33 +74,53 @@ func TestOpenSSLVerifyDetachedSignature(t *testing.T) {
 	}
 
 	// write the root cert to a temp file
-	tmpRootCertFile, err := ioutil.TempFile("", "pkcs7TestRootCA")
+	tmpRootCertFile, err := os.CreateTemp("", "pkcs7TestRootCA")
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer os.Remove(tmpRootCertFile.Name()) // clean up
-	fd, err := os.OpenFile(tmpRootCertFile.Name(), os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0755)
+	defer func(name string) {
+		if err := os.Remove(name); err != nil {
+			t.Fatalf("Cannot write root cert: %s", err)
+		}
+	}(tmpRootCertFile.Name()) // clean up
+	fd, err := os.OpenFile(tmpRootCertFile.Name(), os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0o755)
 	if err != nil {
 		t.Fatal(err)
 	}
-	pem.Encode(fd, &pem.Block{Type: "CERTIFICATE", Bytes: rootCert.Certificate.Raw})
-	fd.Close()
+	if err := pem.Encode(fd, &pem.Block{Type: "CERTIFICATE", Bytes: rootCert.Certificate.Raw}); err != nil {
+		t.Fatalf("Cannot write root cert: %s", err)
+	}
+	if err := fd.Close(); err != nil {
+		t.Fatalf("Cannot write root cert: %s", err)
+	}
 
 	// write the signature to a temp file
-	tmpSignatureFile, err := ioutil.TempFile("", "pkcs7Signature")
+	tmpSignatureFile, err := os.CreateTemp("", "pkcs7Signature")
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer os.Remove(tmpSignatureFile.Name()) // clean up
-	ioutil.WriteFile(tmpSignatureFile.Name(), signed, 0755)
+	defer func(name string) {
+		if err := os.Remove(name); err != nil {
+			t.Fatalf("Cannot write signature: %s", err)
+		}
+	}(tmpSignatureFile.Name()) // clean up
+	if err := os.WriteFile(tmpSignatureFile.Name(), signed, 0o755); err != nil {
+		t.Fatalf("Cannot write signature: %s", err)
+	}
 
 	// write the content to a temp file
-	tmpContentFile, err := ioutil.TempFile("", "pkcs7Content")
+	tmpContentFile, err := os.CreateTemp("", "pkcs7Content")
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer os.Remove(tmpContentFile.Name()) // clean up
-	ioutil.WriteFile(tmpContentFile.Name(), content, 0755)
+	defer func(name string) {
+		if err := os.Remove(name); err != nil {
+			t.Fatalf("Cannot write content: %s", err)
+		}
+	}(tmpContentFile.Name()) // clean up
+	if err := os.WriteFile(tmpContentFile.Name(), content, 0o755); err != nil {
+		t.Fatalf("Cannot write content: %s", err)
+	}
 
 	// call openssl to verify the signature on the content using the root
 	opensslCMD := exec.Command("openssl", "smime", "-verify",
diff --git a/smime.go b/smime.go
index 405a5c1..da09713 100644
--- a/smime.go
+++ b/smime.go
@@ -6,14 +6,14 @@ package mail
 
 import (
 	"bytes"
+	"crypto"
+	"crypto/ecdsa"
 	"crypto/rsa"
 	"crypto/x509"
 	"encoding/pem"
 	"errors"
 	"fmt"
 	"strings"
-
-	"go.mozilla.org/pkcs7"
 )
 
 var (
@@ -22,35 +22,34 @@ var (
 
 	// 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")
-
-	// ErrCouldNotAddSigner should be used if the signer could not be added
-	ErrCouldNotAddSigner = errors.New("could not add signer message")
-
-	// ErrCouldNotFinishSigning should be used if the signing could not be finished
-	ErrCouldNotFinishSigning = errors.New("could not finish signing")
-
-	// ErrCouldNoEncodeToPEM should be used if the signature could not be encoded to PEM
-	ErrCouldNoEncodeToPEM = errors.New("could not encode to PEM")
 )
 
+// privateKeyHolder is the representation of a private key
+type privateKeyHolder struct {
+	ecdsa *ecdsa.PrivateKey
+	rsa   *rsa.PrivateKey
+}
+
+// get returns the private key of the privateKeyHolder
+func (p privateKeyHolder) get() crypto.PrivateKey {
+	if p.ecdsa != nil {
+		return p.ecdsa
+	}
+	return p.rsa
+}
+
 // SMime is used to sign messages with S/MIME
 type SMime struct {
-	privateKey              *rsa.PrivateKey
+	privateKey              privateKeyHolder
 	certificate             *x509.Certificate
 	intermediateCertificate *x509.Certificate
 }
 
-// NewSMime construct a new instance of SMime with provided parameters
+// newSMimeWithRSA 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) {
+// intermediateCertificate (optional) as *x509.Certificate
+func newSMimeWithRSA(privateKey *rsa.PrivateKey, certificate *x509.Certificate, intermediateCertificate *x509.Certificate) (*SMime, error) {
 	if privateKey == nil {
 		return nil, ErrInvalidPrivateKey
 	}
@@ -59,12 +58,28 @@ func newSMime(privateKey *rsa.PrivateKey, certificate *x509.Certificate, interme
 		return nil, ErrInvalidCertificate
 	}
 
-	if intermediateCertificate == nil {
-		return nil, ErrInvalidIntermediateCertificate
+	return &SMime{
+		privateKey:              privateKeyHolder{rsa: privateKey},
+		certificate:             certificate,
+		intermediateCertificate: intermediateCertificate,
+	}, nil
+}
+
+// newSMimeWithECDSA construct a new instance of SMime with provided parameters
+// privateKey as *ecdsa.PrivateKey
+// certificate as *x509.Certificate
+// intermediateCertificate (optional) as *x509.Certificate
+func newSMimeWithECDSA(privateKey *ecdsa.PrivateKey, certificate *x509.Certificate, intermediateCertificate *x509.Certificate) (*SMime, error) {
+	if privateKey == nil {
+		return nil, ErrInvalidPrivateKey
+	}
+
+	if certificate == nil {
+		return nil, ErrInvalidCertificate
 	}
 
 	return &SMime{
-		privateKey:              privateKey,
+		privateKey:              privateKeyHolder{ecdsa: privateKey},
 		certificate:             certificate,
 		intermediateCertificate: intermediateCertificate,
 	}, nil
@@ -75,26 +90,29 @@ func (sm *SMime) signMessage(message string) (*string, error) {
 	lines := parseLines([]byte(message))
 	toBeSigned := lines.bytesFromLines([]byte("\r\n"))
 
-	signedData, err := pkcs7.NewSignedData(toBeSigned)
-	signedData.SetDigestAlgorithm(pkcs7.OIDDigestAlgorithmSHA256)
-	if err != nil {
-		return nil, ErrCouldNotInitialize
+	signedData, err := newSignedData(toBeSigned)
+	if err != nil || signedData == nil {
+		return nil, fmt.Errorf("could not initialize signed data: %w", err)
 	}
 
-	if err = signedData.AddSignerChain(sm.certificate, sm.privateKey, []*x509.Certificate{sm.intermediateCertificate}, pkcs7.SignerInfoConfig{}); err != nil {
-		return nil, ErrCouldNotAddSigner
+	if err = signedData.addSigner(sm.certificate, sm.privateKey.get(), SignerInfoConfig{}); err != nil {
+		return nil, fmt.Errorf("could not add signer message: %w", err)
 	}
 
-	signedData.Detach()
+	if sm.intermediateCertificate != nil {
+		signedData.addCertificate(sm.intermediateCertificate)
+	}
 
-	signatureDER, err := signedData.Finish()
+	signedData.detach()
+
+	signatureDER, err := signedData.finish()
 	if err != nil {
-		return nil, ErrCouldNotFinishSigning
+		return nil, fmt.Errorf("could not finish signing: %w", err)
 	}
 
 	pemMsg, err := encodeToPEM(signatureDER)
 	if err != nil {
-		return nil, ErrCouldNoEncodeToPEM
+		return nil, fmt.Errorf("could not encode to PEM: %w", err)
 	}
 
 	return pemMsg, nil
diff --git a/smime_test.go b/smime_test.go
index 9c5b121..d6fae44 100644
--- a/smime_test.go
+++ b/smime_test.go
@@ -6,25 +6,72 @@ package mail
 
 import (
 	"bytes"
+	"crypto/ecdsa"
+	"crypto/rsa"
 	"encoding/base64"
 	"fmt"
 	"strings"
 	"testing"
 )
 
-// TestNewSMime tests the newSMime method
-func TestNewSMime(t *testing.T) {
-	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
+func TestGet_RSA(t *testing.T) {
+	p := privateKeyHolder{
+		ecdsa: nil,
+		rsa:   &rsa.PrivateKey{},
+	}
+
+	if p.get() == nil {
+		t.Errorf("get() did not return the correct private key")
+	}
+}
+
+func TestGet_ECDSA(t *testing.T) {
+	p := privateKeyHolder{
+		ecdsa: &ecdsa.PrivateKey{},
+		rsa:   nil,
+	}
+
+	if p.get() == nil {
+		t.Errorf("get() did not return the correct private key")
+	}
+}
+
+// TestNewSMimeWithRSA tests the newSMime method with RSA crypto material
+func TestNewSMimeWithRSA(t *testing.T) {
+	privateKey, certificate, intermediateCertificate, err := getDummyRSACryptoMaterial()
 	if err != nil {
 		t.Errorf("Error getting dummy crypto material: %s", err)
 	}
 
-	sMime, err := newSMime(privateKey, certificate, intermediateCertificate)
+	sMime, err := newSMimeWithRSA(privateKey, certificate, intermediateCertificate)
 	if err != nil {
 		t.Errorf("Error creating new SMime from keyPair: %s", err)
 	}
 
-	if sMime.privateKey != privateKey {
+	if sMime.privateKey.rsa != privateKey {
+		t.Errorf("NewSMime() did not return the same private key")
+	}
+	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")
+	}
+}
+
+// TestNewSMimeWithECDSA tests the newSMime method with ECDSA crypto material
+func TestNewSMimeWithECDSA(t *testing.T) {
+	privateKey, certificate, intermediateCertificate, err := getDummyECDSACryptoMaterial()
+	if err != nil {
+		t.Errorf("Error getting dummy crypto material: %s", err)
+	}
+
+	sMime, err := newSMimeWithECDSA(privateKey, certificate, intermediateCertificate)
+	if err != nil {
+		t.Errorf("Error creating new SMime from keyPair: %s", err)
+	}
+
+	if sMime.privateKey.ecdsa != privateKey {
 		t.Errorf("NewSMime() did not return the same private key")
 	}
 	if sMime.certificate != certificate {
@@ -37,12 +84,12 @@ func TestNewSMime(t *testing.T) {
 
 // TestSign tests the sign method
 func TestSign(t *testing.T) {
-	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
+	privateKey, certificate, intermediateCertificate, err := getDummyRSACryptoMaterial()
 	if err != nil {
 		t.Errorf("Error getting dummy crypto material: %s", err)
 	}
 
-	sMime, err := newSMime(privateKey, certificate, intermediateCertificate)
+	sMime, err := newSMimeWithRSA(privateKey, certificate, intermediateCertificate)
 	if err != nil {
 		t.Errorf("Error creating new SMime from keyPair: %s", err)
 	}
@@ -60,12 +107,12 @@ func TestSign(t *testing.T) {
 
 // TestPrepareMessage tests the createMessage method
 func TestPrepareMessage(t *testing.T) {
-	privateKey, certificate, intermediateCertificate, err := getDummyCryptoMaterial()
+	privateKey, certificate, intermediateCertificate, err := getDummyRSACryptoMaterial()
 	if err != nil {
 		t.Errorf("Error getting dummy crypto material: %s", err)
 	}
 
-	sMime, err := newSMime(privateKey, certificate, intermediateCertificate)
+	sMime, err := newSMimeWithRSA(privateKey, certificate, intermediateCertificate)
 	if err != nil {
 		t.Errorf("Error creating new SMime from keyPair: %s", err)
 	}