package smtp

import (
	"errors"
	"io"
)

var (
	ErrAuthRequired    = errors.New("Please authenticate first")
	ErrAuthUnsupported = errors.New("Authentication not supported")
)

// A SMTP server backend.
type Backend interface {
	// Authenticate a user. Return smtp.ErrAuthUnsupported if you don't want to
	// support this.
	Login(username, password string) (User, error)

	// Called if the client attempts to send mail without logging in first.
	// Return smtp.ErrAuthRequired if you don't want to support this.
	AnonymousLogin() (User, error)
}

// An authenticated user.
type User interface {
	// Send an e-mail.
	Send(from string, to []string, r io.Reader) error
	// Logout is called when this User will no longer be used.
	Logout() error
}