Refactor error handling in SMTP authentication

Centralized error definitions in `smtp/auth.go` and updated references in `auth_login.go` and `auth_plain.go`. This improves code maintainability and error consistency across the package.

smtp/auth.go

@@ -13,6 +13,19 @@
 
 package smtp
 
+import "errors"
+
+var (
+	// ErrUnencrypted is an error indicating that the connection is not encrypted.
+	ErrUnencrypted = errors.New("unencrypted connection")
+	// ErrUnexpectedServerChallange is an error indicating that the server issued an unexpected challenge.
+	ErrUnexpectedServerChallange = errors.New("unexpected server challenge")
+	// ErrUnexpectedServerResponse is an error indicating that the server issued an unexpected response.
+	ErrUnexpectedServerResponse = errors.New("unexpected server response")
+	// ErrWrongHostname is an error indicating that the provided hostname does not match the expected value.
+	ErrWrongHostname = errors.New("wrong host name")
+)
+
 // Auth is implemented by an SMTP authentication mechanism.
 type Auth interface {
 	// Start begins an authentication with a server.

smtp/auth_login.go

@@ -5,13 +5,9 @@
 package smtp
 
 import (
-	"errors"
 	"fmt"
 )
 
-// ErrUnencrypted is an error indicating that the connection is not encrypted.
-var ErrUnencrypted = errors.New("unencrypted connection")
-
 // loginAuth is the type that satisfies the Auth interface for the "SMTP LOGIN" auth
 type loginAuth struct {
 	username, password string
@@ -55,7 +51,7 @@ func (a *loginAuth) Start(server *ServerInfo) (string, []byte, error) {
 		return "", nil, ErrUnencrypted
 	}
 	if server.Name != a.host {
-		return "", nil, errors.New("wrong host name")
+		return "", nil, ErrWrongHostname
 	}
 	a.respStep = 0
 	return "LOGIN", nil, nil
@@ -73,7 +69,7 @@ func (a *loginAuth) Next(fromServer []byte, more bool) ([]byte, error) {
 			a.respStep++
 			return []byte(a.password), nil
 		default:
-			return nil, fmt.Errorf("unexpected server response: %s", string(fromServer))
+			return nil, fmt.Errorf("%w: %s", ErrUnexpectedServerResponse, string(fromServer))
 		}
 	}
 	return nil, nil

smtp/auth_plain.go

@@ -13,10 +13,6 @@
 
 package smtp
 
-import (
-	"errors"
-)
-
 // plainAuth is the type that satisfies the Auth interface for the "SMTP PLAIN" auth
 type plainAuth struct {
 	identity, username, password string
@@ -42,10 +38,10 @@ func (a *plainAuth) Start(server *ServerInfo) (string, []byte, error) {
 	// That might just be the attacker saying
 	// "it's ok, you can trust me with your password."
 	if !server.TLS && !isLocalhost(server.Name) {
-		return "", nil, errors.New("unencrypted connection")
+		return "", nil, ErrUnencrypted
 	}
 	if server.Name != a.host {
-		return "", nil, errors.New("wrong host name")
+		return "", nil, ErrWrongHostname
 	}
 	resp := []byte(a.identity + "\x00" + a.username + "\x00" + a.password)
 	return "PLAIN", resp, nil
@@ -54,7 +50,7 @@ func (a *plainAuth) Start(server *ServerInfo) (string, []byte, error) {
 func (a *plainAuth) Next(_ []byte, more bool) ([]byte, error) {
 	if more {
 		// We've already sent everything.
-		return nil, errors.New("unexpected server challenge")
+		return nil, ErrUnexpectedServerChallange
 	}
 	return nil, nil
 }