Attribute sMimeSinging was added in msg that indicates whether the message should be singed with S/MIME when it's sent. Also, sMimeAuthConfig was introduced in client so that required privateKey and certificate can be used for S/MIME signing.

client.go

@@ -127,6 +127,9 @@ type Client struct {
 	// smtpAuthType represents the authentication type for SMTP AUTH
 	smtpAuthType SMTPAuthType
 
+	// SMimeAuthConfig represents the authentication type for s/mime crypto key material
+	sMimeAuthConfig *SMimeAuthConfig
+
 	// smtpClient is the smtp.Client that is set up when using the Dial*() methods
 	smtpClient *smtp.Client
 
@@ -168,6 +171,9 @@ var (
 	// ErrInvalidTLSConfig should be used if an empty tls.Config is provided
 	ErrInvalidTLSConfig = errors.New("invalid TLS config")
 
+	// ErrInvalidSMimeAuthConfig should be used if the values in the struct SMimeAuthConfig are empty
+	ErrInvalidSMimeAuthConfig = errors.New("invalid S/MIME authentication config")
+
 	// ErrNoHostname should be used if a Client has no hostname set
 	ErrNoHostname = errors.New("hostname for client cannot be empty")
 
@@ -459,6 +465,17 @@ func WithDialContextFunc(dialCtxFunc DialContextFunc) Option {
 	}
 }
 
+// WithSMimeConfig tells the client to use the provided SMIMEAuth for s/mime crypto key material authentication
+func WithSMimeConfig(sMimeAuthConfig *SMimeAuthConfig) Option {
+	return func(c *Client) error {
+		if sMimeAuthConfig.Certificate == nil && sMimeAuthConfig.PrivateKey == nil {
+			return ErrInvalidSMimeAuthConfig
+		}
+		c.sMimeAuthConfig = sMimeAuthConfig
+		return nil
+	}
+}
+
 // TLSPolicy returns the currently set TLSPolicy as string
 func (c *Client) TLSPolicy() string {
 	return c.tlspolicy.String()

client_test.go

@@ -6,10 +6,15 @@ package mail
 
 import (
 	"context"
+	"crypto/rand"
+	"crypto/rsa"
 	"crypto/tls"
+	"crypto/x509"
+	"crypto/x509/pkix"
 	"errors"
 	"fmt"
 	"io"
+	"math/big"
 	"net"
 	"os"
 	"strconv"
@@ -117,7 +122,7 @@ func TestNewClientWithOptions(t *testing.T) {
 		{"WithDialContextFunc()", WithDialContextFunc(func(ctx context.Context, network, address string) (net.Conn, error) {
 			return nil, nil
 		}), false},
-
+		{"WithSMimeConfig()", WithSMimeConfig(&SMimeAuthConfig{}), true},
 		{
 			"WithDSNRcptNotifyType() NEVER combination",
 			WithDSNRcptNotifyType(DSNRcptNotifySuccess, DSNRcptNotifyNever), true,
@@ -756,6 +761,43 @@ func TestClient_DialWithContextInvalidAuth(t *testing.T) {
 	}
 }
 
+// TestWithSMime tests the WithSMime method with invalid SMimeAuthConfig for the Client object
+func TestWithSMime_InvalidConfig(t *testing.T) {
+	_, err := NewClient(DefaultHost, WithSMimeConfig(&SMimeAuthConfig{}))
+	if !errors.Is(err, ErrInvalidSMimeAuthConfig) {
+		t.Errorf("failed to check sMimeAuthConfig values correctly: %s", err)
+	}
+}
+
+// TestWithSMime tests the WithSMime method with valid SMimeAuthConfig that is loaded from dummy certificate for the Client object
+func TestWithSMime_ValidConfig(t *testing.T) {
+	privateKey, err := getDummyPrivateKey()
+	if err != nil {
+		t.Errorf("failed to load dummy private key: %s", err)
+	}
+
+	certificate, err := getDummyCertificate(privateKey)
+	if err != nil {
+		t.Errorf("failed to load dummy certificate: %s", err)
+	}
+
+	sMimeAuthConfig := &SMimeAuthConfig{PrivateKey: privateKey, Certificate: certificate}
+	c, err := NewClient(DefaultHost, WithSMimeConfig(sMimeAuthConfig))
+	if err != nil {
+		t.Errorf("failed to create new client: %s", err)
+	}
+
+	if c.sMimeAuthConfig != sMimeAuthConfig {
+		t.Errorf("failed to set smeAuthConfig. Expected %v, got: %v", sMimeAuthConfig, c.sMimeAuthConfig)
+	}
+	if c.sMimeAuthConfig.PrivateKey != sMimeAuthConfig.PrivateKey {
+		t.Errorf("failed to set smeAuthConfig.PrivateKey Expected %v, got: %v", sMimeAuthConfig, c.sMimeAuthConfig)
+	}
+	if c.sMimeAuthConfig.Certificate != sMimeAuthConfig.Certificate {
+		t.Errorf("failed to set smeAuthConfig.Certificate Expected %v, got: %v", sMimeAuthConfig, c.sMimeAuthConfig)
+	}
+}
+
 // TestClient_checkConn tests the checkConn method with intentional breaking for the Client object
 func TestClient_checkConn(t *testing.T) {
 	c, err := getTestConnection(true)
@@ -1484,6 +1526,37 @@ func getFakeDialFunc(conn net.Conn) DialContextFunc {
 	}
 }
 
+func getDummyPrivateKey() (*rsa.PrivateKey, error) {
+	privateKey, err := rsa.GenerateKey(rand.Reader, 2048)
+	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
+}
+
 type faker struct {
 	io.ReadWriter
 }

msg.go

@@ -120,6 +120,9 @@ type Msg struct {
 
 	// noDefaultUserAgent indicates whether the default User Agent will be excluded for the Msg when it's sent.
 	noDefaultUserAgent bool
+
+	// sMimeSinging indicates whether the message should be singed with S/MIME when it's sent.
+	sMimeSinging bool
 }
 
 // SendmailPath is the default system path to the sendmail binary
@@ -202,6 +205,13 @@ func WithNoDefaultUserAgent() MsgOption {
 	}
 }
 
+// WithSMimeSinging configures the Msg to be S/MIME singed sent.
+func WithSMimeSinging() MsgOption {
+	return func(m *Msg) {
+		m.sMimeSinging = true
+	}
+}
+
 // SetCharset sets the encoding charset of the Msg
 func (m *Msg) SetCharset(c Charset) {
 	m.charset = c

msg_test.go

@@ -3233,6 +3233,14 @@ func TestNewMsgWithNoDefaultUserAgent(t *testing.T) {
 	}
 }
 
+// TestWithSMimeSinging tests WithSMimeSinging
+func TestWithSMimeSinging(t *testing.T) {
+	m := NewMsg(WithSMimeSinging())
+	if m.sMimeSinging != true {
+		t.Errorf("WithSMimeSinging() failed. Expected: %t, got: %t", true, false)
+	}
+}
+
 // Fuzzing tests
 func FuzzMsg_Subject(f *testing.F) {
 	f.Add("Testsubject")

sime.go

@@ -0,0 +1,12 @@
+package mail
+
+import (
+	"crypto/rsa"
+	"crypto/x509"
+)
+
+// SMimeAuthConfig represents the authentication type for s/mime crypto key material
+type SMimeAuthConfig struct {
+	Certificate *x509.Certificate
+	PrivateKey  *rsa.PrivateKey
+}