POP/IMAP-before-SMTP

POP/IMAP-before-SMTP

為了防止垃圾郵件，大部分郵件伺服器在發送郵件的前都會對用戶端進行身分識別驗證。主要的驗證方式有兩種：
1.
SMTP AUTH: 對SMTP協議進行擴充，使用EHELO和AUTH命令來互動使用者名和密碼。請參考《SMTP驗證》
一文。
2.
POP/IMAP-before-SMTP: 在發送郵件之前，先使用POP/IMAP協議驗證身份。

SMTP
AUTH是對SMTP協議的擴充，而POP/IMAP-before-SMTP是對郵件伺服器處理流程的簡單擴充：當郵件用戶端使用POP/IMAP接受郵件時，如果驗證通過，伺服器會把用戶端的IP地址記錄在本地的臨時檔案中；然後郵件用戶端使用SMTP發送郵件，伺服器會在本地IP地址臨時檔案中進行搜尋，如果可以找到，則繼續後面的操作，否則會拒絕用戶端的請求。伺服器會週期性清理IP地址臨時檔案中的到期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.