Add support for PLAIN authentication without encryption

Implemented a new SMTPAuthPlainNoEnc option to allow PLAIN authentication over unencrypted connections. Refactored the PlainAuth function to accept an additional allowUnencryptedAuth parameter. Updated relevant tests to cover the new authentication method.

auth.go

@@ -62,6 +62,20 @@ const (
 	// https://datatracker.ietf.org/doc/html/rfc4616/
 	SMTPAuthPlain SMTPAuthType = "PLAIN"
 
+	// SMTPAuthPlainNoEnc is the "PLAIN" authentication mechanism as described in RFC 4616.
+	//
+	// Since the "PLAIN" SASL authentication mechanism transmits the username and password in
+	// plaintext over the internet connection, bye default we only allow this mechanism over
+	// a TLS secured connection. This authentiation mechanism overrides this default and will
+	// allow PLAIN authentication via an unencrypted channel. This can be useful if the
+	// connection has already been secured in a different way (e. g. a SSH tunnel)
+	//
+	// Note: Use this authentication method with caution. If used in the wrong way, you might
+	// expose your authentication information over unencrypted channels!
+	//
+	// https://datatracker.ietf.org/doc/html/rfc4616/
+	SMTPAuthPlainNoEnc SMTPAuthType = "PLAIN-NOENC"
+
 	// SMTPAuthXOAUTH2 is the "XOAUTH2" SASL authentication mechanism.
 	// https://developers.google.com/gmail/imap/xoauth2-protocol
 	SMTPAuthXOAUTH2 SMTPAuthType = "XOAUTH2"

client.go

@@ -1096,7 +1096,12 @@ func (c *Client) auth() error {
 			if !strings.Contains(smtpAuthType, string(SMTPAuthPlain)) {
 				return ErrPlainAuthNotSupported
 			}
-			c.smtpAuth = smtp.PlainAuth("", c.user, c.pass, c.host)
+			c.smtpAuth = smtp.PlainAuth("", c.user, c.pass, c.host, false)
+		case SMTPAuthPlainNoEnc:
+			if !strings.Contains(smtpAuthType, string(SMTPAuthPlain)) {
+				return ErrPlainAuthNotSupported
+			}
+			c.smtpAuth = smtp.PlainAuth("", c.user, c.pass, c.host, true)
 		case SMTPAuthLogin:
 			if !strings.Contains(smtpAuthType, string(SMTPAuthLogin)) {
 				return ErrLoginAuthNotSupported

client_test.go

@@ -110,7 +110,7 @@ func TestNewClientWithOptions(t *testing.T) {
 		{"WithSMTPAuth()", WithSMTPAuth(SMTPAuthLogin), false},
 		{
 			"WithSMTPAuthCustom()",
-			WithSMTPAuthCustom(smtp.PlainAuth("", "", "", "")),
+			WithSMTPAuthCustom(smtp.PlainAuth("", "", "", "", false)),
 			false,
 		},
 		{"WithUsername()", WithUsername("test"), false},
@@ -606,7 +606,7 @@ func TestSetSMTPAuthCustom(t *testing.T) {
 	}{
 		{"SMTPAuth: CRAM-MD5", smtp.CRAMMD5Auth("", ""), "CRAM-MD5", false},
 		{"SMTPAuth: LOGIN", smtp.LoginAuth("", "", ""), "LOGIN", false},
-		{"SMTPAuth: PLAIN", smtp.PlainAuth("", "", "", ""), "PLAIN", false},
+		{"SMTPAuth: PLAIN", smtp.PlainAuth("", "", "", "", false), "PLAIN", false},
 	}
 	si := smtp.ServerInfo{TLS: true}
 	for _, tt := range tests {
@@ -1227,7 +1227,7 @@ func TestClient_DialWithContext_switchAuth(t *testing.T) {
 
 	// We switch to CUSTOM by providing PLAIN auth as function - the server supports this
 	client.SetSMTPAuthCustom(smtp.PlainAuth("", os.Getenv("TEST_SMTPAUTH_USER"),
-		os.Getenv("TEST_SMTPAUTH_PASS"), os.Getenv("TEST_HOST")))
+		os.Getenv("TEST_SMTPAUTH_PASS"), os.Getenv("TEST_HOST"), false))
 	if client.smtpAuthType != SMTPAuthCustom {
 		t.Errorf("expected auth type to be Custom, got: %s", client.smtpAuthType)
 	}

smtp/auth_plain.go

@@ -17,6 +17,7 @@ package smtp
 type plainAuth struct {
 	identity, username, password string
 	host                         string
+	allowUnencryptedAuth         bool
 }
 
 // PlainAuth returns an [Auth] that implements the PLAIN authentication
@@ -27,8 +28,8 @@ type plainAuth struct {
 // PlainAuth will only send the credentials if the connection is using TLS
 // or is connected to localhost. Otherwise authentication will fail with an
 // error, without sending the credentials.
-func PlainAuth(identity, username, password, host string) Auth {
+func PlainAuth(identity, username, password, host string, allowUnEnc bool) Auth {
-	return &plainAuth{identity, username, password, host}
+	return &plainAuth{identity, username, password, host, allowUnEnc}
 }
 
 func (a *plainAuth) Start(server *ServerInfo) (string, []byte, error) {
@@ -37,7 +38,7 @@ func (a *plainAuth) Start(server *ServerInfo) (string, []byte, error) {
 	// In particular, it doesn't matter if the server advertises PLAIN auth.
 	// That might just be the attacker saying
 	// "it's ok, you can trust me with your password."
-	if !server.TLS && !isLocalhost(server.Name) {
+	if !a.allowUnencryptedAuth && !server.TLS && !isLocalhost(server.Name) {
 		return "", nil, ErrUnencrypted
 	}
 	if server.Name != a.host {