邮件客户端通过SMTP，将邮件发送到邮件服务器，之后邮件服务器再将邮件发送到目标用户的电子邮箱。SMTP是一种请求/响应协议，邮件客户端发出一个命令，邮件服务器处理该命令，然后给邮件客户端发回一个回复。-Mail client through SMTP, the mail is sent to the mail server, mail server and then after the e-mail to the target user s e-mail address. SMTP is a request/response protocol, e-mail client sends a command, the mail server handling the command, and then to the mail client sent back a reply.