Reset response step in AUTH LOGIN initialization

The addition of `a.respStep = 0` resets the response step counter at the beginning of the AUTH LOGIN process. This ensures that the state starts correctly and avoids potential issues related to residual values from previous authentications.

smtp/auth_login.go

@@ -57,6 +57,7 @@ func (a *loginAuth) Start(server *ServerInfo) (string, []byte, error) {
 	if server.Name != a.host {
 		return "", nil, errors.New("wrong host name")
 	}
+	a.respStep = 0
 	return "LOGIN", nil, nil
 }