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POP/IMAP-before-SMTP

为了防止垃圾邮件，大部分邮件服务器在发送邮件的前都会对客户端进行身份验证。主要的验证方式有两种： 1. SMTP AUTH: 对SMTP协议进行扩展，使用EHELO和AUTH命令来交互用户名和密码。请参考 一文。 2. POP/IMAP-before-SMTP: 在发送邮件之前，先使用POP/IMAP协议验证身份。 SMTP AUTH是对SMTP协议的扩展，而POP/IMAP-before-SMTP是对邮件服务器处理流程的简单扩展：当邮件客户端使用POP/IMAP接受邮件时，如果验证通过，服务器会把客户端的IP地址记录在本地的临时文件中；然后邮件客户端使用SMTP发送邮件，服务器会在本地IP地址临时文件中进行搜索，如果可以找到，则继续后面的操作，否则会拒绝客户端的请求。服务器会定期的清理IP地址临时文件中的过期IP。
What is POP before SMTP? Answer: POP before SMTP is a means of allowing an external user to use your mail server to send external email if and only if they have been authenticated by the POP service on your server first. It does not matter what IP address they use providing they can be authenticated by username and password. Once the user has been successfully validated, their IP address is temporarily seen as "local" and thus the user will now be able to use your server as an outgoing mail relay. The user connecting with this IP is able to relay mail for a time period setup by the administrator. What is IMAP before SMTP? Answer: IMAP before SMTP is a means of allowing an external user to use your mail server to send external email if and only if they have been authenticated by the IMAP service on your server first. It does not matter what IP address they log on from providing they can be authenticated by username and password.

 

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