diff --git a/go.mod b/go.mod
index b155b9b..a59f78a 100644
--- a/go.mod
+++ b/go.mod
@@ -5,3 +5,5 @@
 module github.com/wneessen/go-mail
 
 go 1.16
+
+require go.mozilla.org/pkcs7 v0.9.0
diff --git a/go.sum b/go.sum
index e69de29..4ca3691 100644
--- a/go.sum
+++ b/go.sum
@@ -0,0 +1,2 @@
+go.mozilla.org/pkcs7 v0.9.0 h1:yM4/HS9dYv7ri2biPtxt8ikvB37a980dg69/pKmS+eI=
+go.mozilla.org/pkcs7 v0.9.0/go.mod h1:SNgMg+EgDFwmvSmLRTNKC5fegJjB7v23qTQ0XLGUNHk=
diff --git a/msg.go b/msg.go
index 24e2ca6..9686dac 100644
--- a/msg.go
+++ b/msg.go
@@ -7,6 +7,8 @@ package mail
 import (
 	"bytes"
 	"context"
+	"crypto/rsa"
+	"crypto/x509"
 	"embed"
 	"errors"
 	"fmt"
@@ -981,10 +983,47 @@ func (m *Msg) applyMiddlewares(msg *Msg) *Msg {
 	return msg
 }
 
+// signMessage sign the Msg with S/MIME
+func (m *Msg) signMessage(msg *Msg) (*Msg, error) {
+	currentPart := m.GetParts()[0]
+	currentPart.SetEncoding(EncodingUSASCII)
+	currentPart.SetContentType(TypeTextPlain)
+	content, err := currentPart.GetContent()
+	if err != nil {
+		return nil, errors.New("failed to extract content from part")
+	}
+
+	signedContent, err := m.sMime.Sign(content)
+	if err != nil {
+		return nil, errors.New("failed to sign message")
+	}
+
+	signedPart := msg.newPart(
+		typeSMimeSigned,
+		WithPartEncoding(EncodingB64),
+		WithContentDisposition(DispositionSMime),
+	)
+	signedPart.SetContent(*signedContent)
+	msg.parts = append(msg.parts, signedPart)
+
+	return msg, nil
+}
+
 // WriteTo writes the formated Msg into a give io.Writer and satisfies the io.WriteTo interface
 func (m *Msg) WriteTo(writer io.Writer) (int64, error) {
 	mw := &msgWriter{writer: writer, charset: m.charset, encoder: m.encoder}
-	mw.writeMsg(m.applyMiddlewares(m))
+	msg := m.applyMiddlewares(m)
+
+	if m.sMime != nil {
+		signedMsg, err := m.signMessage(msg)
+		if err != nil {
+			return 0, err
+		}
+		msg = signedMsg
+	}
+
+	mw.writeMsg(msg)
+
 	return mw.bytesWritten, mw.err
 }
 
diff --git a/msgwriter.go b/msgwriter.go
index ff1b47b..684c223 100644
--- a/msgwriter.go
+++ b/msgwriter.go
@@ -88,6 +88,10 @@ func (mw *msgWriter) writeMsg(msg *Msg) {
 		}
 	}
 
+	if msg.hasSMime() {
+		mw.startMP(MIMESMime, msg.boundary)
+		mw.writeString(DoubleNewLine)
+	}
 	if msg.hasMixed() {
 		mw.startMP(MIMEMixed, msg.boundary)
 		mw.writeString(DoubleNewLine)
@@ -96,7 +100,7 @@ func (mw *msgWriter) writeMsg(msg *Msg) {
 		mw.startMP(MIMERelated, msg.boundary)
 		mw.writeString(DoubleNewLine)
 	}
-	if msg.hasAlt() {
+	if msg.hasAlt() && !msg.hasSMime() {
 		mw.startMP(MIMEAlternative, msg.boundary)
 		mw.writeString(DoubleNewLine)
 	}
@@ -265,6 +269,9 @@ func (mw *msgWriter) writePart(part *Part, charset Charset) {
 		if part.description != "" {
 			mimeHeader.Add(string(HeaderContentDescription), part.description)
 		}
+		if part.disposition != "" {
+			mimeHeader.Add(string(HeaderContentDisposition), part.disposition.String())
+		}
 		mimeHeader.Add(string(HeaderContentType), contentType)
 		mimeHeader.Add(string(HeaderContentTransferEnc), contentTransferEnc)
 		mw.newPart(mimeHeader)
diff --git a/sime.go b/sime.go
index 714d3a4..f230a69 100644
--- a/sime.go
+++ b/sime.go
@@ -3,10 +3,68 @@ package mail
 import (
 	"crypto/rsa"
 	"crypto/x509"
+	"errors"
+	"go.mozilla.org/pkcs7"
 )
 
-// SMimeAuthConfig represents the authentication type for s/mime crypto key material
-type SMimeAuthConfig struct {
-	Certificate *x509.Certificate
-	PrivateKey  *rsa.PrivateKey
+var (
+	// ErrInvalidPrivateKey should be used if private key is invalid
+	ErrInvalidPrivateKey = errors.New("invalid private key")
+
+	// ErrInvalidCertificate should be used if certificate is invalid
+	ErrInvalidCertificate = errors.New("invalid 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")
+)
+
+// SMime is used to sign messages with S/MIME
+type SMime struct {
+	privateKey  *rsa.PrivateKey
+	certificate *x509.Certificate
+}
+
+// NewSMime construct a new instance of SMime with a provided *rsa.PrivateKey
+func NewSMime(privateKey *rsa.PrivateKey, certificate *x509.Certificate) (*SMime, error) {
+	if privateKey == nil {
+		return nil, ErrInvalidPrivateKey
+	}
+
+	if certificate == nil {
+		return nil, ErrInvalidCertificate
+	}
+
+	return &SMime{
+		privateKey:  privateKey,
+		certificate: certificate,
+	}, nil
+}
+
+// Sign the content with the given privateKey of the method NewSMime
+func (sm *SMime) Sign(content []byte) (*string, error) {
+	toBeSigned, err := pkcs7.NewSignedData(content)
+
+	toBeSigned.SetDigestAlgorithm(pkcs7.OIDDigestAlgorithmSHA256)
+	if err != nil {
+		return nil, ErrCouldNotInitialize
+	}
+
+	if err = toBeSigned.AddSigner(sm.certificate, sm.privateKey, pkcs7.SignerInfoConfig{}); err != nil {
+		return nil, ErrCouldNotAddSigner
+	}
+
+	signed, err := toBeSigned.Finish()
+	if err != nil {
+		return nil, ErrCouldNotFinishSigning
+	}
+
+	signedData := string(signed)
+
+	return &signedData, nil
 }