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协议参考

• https://tools.ietf.org/html/rfc5321
• https://www.rfc-editor.org/rfc/rfc5321.txt

SMTP设计图如下：

              +----------+                +----------+
  +------+    |          |                |          |
  | User |<-->|          |      SMTP      |          |
  +------+    |  Client- |Commands/Replies| Server-  |
  +------+    |   SMTP   |<-------------->|    SMTP  |    +------+
  | File |<-->|          |    and Mail    |          |<-->| File |
  |System|    |          |                |          |    |System|
  +------+    +----------+                +----------+    +------+
               SMTP client                SMTP server

mail.qq.com | cmd | 例子

midoks@midoks ~ % telnet smtp.qq.com 25
Trying 240e:ff:f100:8019::6a...
Connected to smtp.qq.com.
Escape character is '^]'.
220 newxmesmtplogicsvrsza9.qq.com XMail Esmtp QQ Mail Server.
helo m
250-newxmesmtplogicsvrsza9.qq.com-9.21.152.27-23666742
250-SIZE 73400320
250 OK
auth login
334 VXNlcm5hbWU6
用户    ---[base64]
334 UGFzc3dvcmQ6
授权码    ---[base64]
235 Authentication successful
mail from:<627293072@qq.com>
250 OK.
rcpt to: <midoks@163.com>
250 OK
data
354 End data with <CR><LF>.<CR><LF>.
from:627293072
to: midoks
subject: test

data
.
250 OK: queued as.

localhost | cmd | debug

midoks@midoks ~ % telnet 127.0.0.1 25
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 Anti-spam GT for Coremail System(imail)
helo m
250 ok
auth login 
334 dXNlcm5hbWU6
YWRtaW4=
334 UGFzc3dvcmQ6
YWRtaW4=
235 Authentication successful
mail from:<admin@xxx.com>
250 Mail OK
rcpt to:<midoks@163.com>
250 Mail OK
data
354 End data with <CR><LF>.<CR><LF>