#332: Add default SMTP authentication type to NewClient

This commit fixes a regression introduced in v0.5.0. We now set the default SMTPAuthType to NOAUTH in NewClient. Enhanced documentation and added test cases for different SMTP authentication scenarios.

auth.go

@@ -47,7 +47,7 @@ const (
 	// SMTPAuthNoAuth is equivalent to performing no authentication at all. It is a convenience
 	// option and should not be used. Instead, for mail servers that do no support/require
 	// authentication, the Client should not be passed the WithSMTPAuth option at all.
-	SMTPAuthNoAuth SMTPAuthType = ""
+	SMTPAuthNoAuth SMTPAuthType = "NOAUTH"
 
 	// SMTPAuthPlain is the "PLAIN" authentication mechanism as described in RFC 4616.
 	//

client.go

@@ -256,6 +256,7 @@ var (
 //   - An error if any critical default values are missing or options fail to apply.
 func NewClient(host string, opts ...Option) (*Client, error) {
 	c := &Client{
+		smtpAuthType: SMTPAuthNoAuth,
 		connTimeout:  DefaultTimeout,
 		host:         host,
 		port:         DefaultPort,
@@ -1028,9 +1029,16 @@ func (c *Client) DialAndSendWithContext(ctx context.Context, messages ...*Msg) e
 // determines the supported authentication methods, and applies the appropriate authentication
 // type. An error is returned if authentication fails.
 //
-// This method first verifies the connection to the SMTP server. If no custom authentication
-// mechanism is provided, it checks which authentication methods are supported by the server.
-// Based on the configured SMTPAuthType, it sets up the appropriate authentication mechanism.
+// By default NewClient sets the SMTP authentication type to SMTPAuthNoAuth, meaning, that no
+// SMTP authentication will be performed. If the user makes use of SetSMTPAuth or initialzes the
+// client with WithSMTPAuth, the SMTP authentication type will be set in the Client, forcing
+// this method to determine if the server supports the selected authentication method and
+// assigning the corresponding smtp.Auth function to it.
+//
+// If the user set a custom SMTP authentication function using SetSMTPAuthCustom or
+// WithSMTPAuthCustom, we will not perform any detection and assignment logic and will trust
+// the user with their provided smtp.Auth function.
+//
 // Finally, it attempts to authenticate the client using the selected method.
 //
 // Returns:
@@ -1040,7 +1048,8 @@ func (c *Client) auth() error {
 	if err := c.checkConn(); err != nil {
 		return fmt.Errorf("failed to authenticate: %w", err)
 	}
-	if c.smtpAuth == nil && c.smtpAuthType != SMTPAuthCustom {
+
+	if c.smtpAuth == nil && c.smtpAuthType != SMTPAuthNoAuth {
 		hasSMTPAuth, smtpAuthType := c.smtpClient.Extension("AUTH")
 		if !hasSMTPAuth {
 			return fmt.Errorf("server does not support SMTP AUTH")

client_test.go

@@ -1162,6 +1162,81 @@ func TestClient_Send_withBrokenRecipient(t *testing.T) {
 	}
 }
 
+func TestClient_DialWithContext_switchAuth(t *testing.T) {
+	if os.Getenv("TEST_ALLOW_SEND") == "" {
+		t.Skipf("TEST_ALLOW_SEND is not set. Skipping mail sending test")
+	}
+
+	// We start with no auth explicitly set
+	client, err := NewClient(
+		os.Getenv("TEST_HOST"),
+		WithTLSPortPolicy(TLSMandatory),
+	)
+	defer func() {
+		_ = client.Close()
+	}()
+	if err != nil {
+		t.Errorf("failed to create client: %s", err)
+		return
+	}
+	if err = client.DialWithContext(context.Background()); err != nil {
+		t.Errorf("failed to dial to sending server: %s", err)
+	}
+	if err = client.Close(); err != nil {
+		t.Errorf("failed to close client connection: %s", err)
+	}
+
+	// We switch to LOGIN auth, which the server supports
+	client.SetSMTPAuth(SMTPAuthLogin)
+	client.SetUsername(os.Getenv("TEST_USER"))
+	client.SetPassword(os.Getenv("TEST_PASS"))
+	if err = client.DialWithContext(context.Background()); err != nil {
+		t.Errorf("failed to dial to sending server: %s", err)
+	}
+	if err = client.Close(); err != nil {
+		t.Errorf("failed to close client connection: %s", err)
+	}
+
+	// We switch to CRAM-MD5, which the server does not support - error expected
+	client.SetSMTPAuth(SMTPAuthCramMD5)
+	if err = client.DialWithContext(context.Background()); err == nil {
+		t.Errorf("expected error when dialing with unsupported auth mechanism, got nil")
+		return
+	}
+	if !errors.Is(err, ErrCramMD5AuthNotSupported) {
+		t.Errorf("expected dial error: %s, but got: %s", ErrCramMD5AuthNotSupported, err)
+	}
+
+	// We switch to CUSTOM by providing PLAIN auth as function - the server supports this
+	client.SetSMTPAuthCustom(smtp.PlainAuth("", os.Getenv("TEST_USER"), os.Getenv("TEST_PASS"),
+		os.Getenv("TEST_HOST")))
+	if client.smtpAuthType != SMTPAuthCustom {
+		t.Errorf("expected auth type to be Custom, got: %s", client.smtpAuthType)
+	}
+	if err = client.DialWithContext(context.Background()); err != nil {
+		t.Errorf("failed to dial to sending server: %s", err)
+	}
+	if err = client.Close(); err != nil {
+		t.Errorf("failed to close client connection: %s", err)
+	}
+
+	// We switch back to explicit no authenticaiton
+	client.SetSMTPAuth(SMTPAuthNoAuth)
+	if err = client.DialWithContext(context.Background()); err != nil {
+		t.Errorf("failed to dial to sending server: %s", err)
+	}
+	if err = client.Close(); err != nil {
+		t.Errorf("failed to close client connection: %s", err)
+	}
+
+	// Finally we set an empty string as SMTPAuthType and expect and error. This way we can
+	// verify that we do not accidentaly skip authentication with an empty string SMTPAuthType
+	client.SetSMTPAuth("")
+	if err = client.DialWithContext(context.Background()); err == nil {
+		t.Errorf("expected error when dialing with empty auth mechanism, got nil")
+	}
+}
+
 // TestClient_auth tests the Dial(), Send() and Close() method of Client with broken settings
 func TestClient_auth(t *testing.T) {
 	tests := []struct {