SMTP（简单邮件传输协议）协议是一种邮件发送协议，他的目标是可靠、高效的传送邮件，他独立于传送子系统，而且仅要求一条可以保证传送数据单元顺序的通道。SMTP协议的一个重要特点是它能够在传送中接力传送邮件。也就是说，如果该邮件服务器知道目的地址的邮件服务器则直接发送；如果不知道目的邮件服务器，则将这封信件按照某种策略转发给就近的一个邮件服务器。通过这种接力式的传送方式，完成邮件的发送-SMTP (Simple Mail Transfer Protocol) protocol is an e-mail agreement, his goal is a reliable and efficient transmission of the message, he independent of the transmission subsystem and requires only a transmission of data to ensure that the order of the channel unit. SMTP protocol is an important characteristic that it can send a message relay transmission. In other words, if the mail server know the destination address of the mail server directly to send　If you do not know the purpose of the mail server will be a strategy in accordance with the letter transmitted to a nearby mail server. Relay through this type of delivery to complete the e-mail sender