bborn 你可以试想一下两个邮箱之间是怎么通信的 现在不用的原因 我想主要是为了防止垃圾邮件

感谢他的提示，我找到RFC http://www.faqs.org/rfcs/rfc821.html

A Typical SMTP Transaction Scenario

      This SMTP example shows mail sent by Smith at host USC-ISIF, to
      Jones, Green, and Brown at host BBN-UNIX.  Here we assume that
      host USC-ISIF contacts host BBN-UNIX directly.  The mail is
      accepted for Jones and Brown.  Green does not have a mailbox at
      host BBN-UNIX.

      -------------------------------------------------------------

         R: 220 BBN-UNIX.ARPA Simple Mail Transfer Service Ready
         S: HELO USC-ISIF.ARPA
         R: 250 BBN-UNIX.ARPA

         S: MAIL FROM:<Smith at USC-ISIF dot ARPA>
         R: 250 OK

         S: RCPT TO:<Jones at BBN-UNIX dot ARPA>
         R: 250 OK

         S: RCPT TO:<Green at BBN-UNIX dot ARPA>
         R: 550 No such user here

         S: RCPT TO:<Brown at BBN-UNIX dot ARPA>
         R: 250 OK

         S: DATA
         R: 354 Start mail input; end with <CRLF>.<CRLF>
         S: Blah blah blah...
         S: ...etc. etc. etc.
         S: .
         R: 250 OK

         S: QUIT
         R: 221 BBN-UNIX.ARPA Service closing transmission channel

根据它的格式，使用公司内部SMTP服务器给自己发送了一份EMAIL成功，

可是我试验 163.COM和SINA.COM却都失败了 ：）

220 tanis7.sina.com ESMTP Postfix
helo 163.com
250 tanis7.sina.com
mail from:<sf7509 at 163 dot com>
250 Ok
rcpt to:<sf7509 at sina dot com>
554 <sf7509 at sina dot com>: Relay access denied

data
554 Error: no valid recipients

rcpt to:<sf7509 at sina dot com>
554 <sf7509 at sina dot com>: Relay access denied
rcpt to:<sf7509 at 163 dot com>
554 <sf7509 at 163 dot com>: Relay access denied

220 Coremail SMTP(Anti Spam) System (163com[20050206])
helo sina.com
250 mx7
mail from:<sf7509 at sina dot com>
550 Reject by MX Filter BlackList(LQD90rtc00L3x+kC.1.X.31)

如果我使用公司的帐号就OK了，如果使用乱七八糟的一个：

220 Coremail SMTP(Anti Spam) System (163com[20050206])
helo sf.net
250 mx10
mail from:<sf at sf dot net>
250 Ok
rcpt to:<sf7509 at 163 dot com>
250 Ok
data
354 End data with <CR><LF>.<CR><LF>
ipeurt
ergdfg
egdfg
ergerg
.
250 Ok: queued as MoCRk+Bc00JPNpYB.1

http://www.shengfang.org/blog/p/b2smtpdesign2.php

真是很奇怪，按照常规来说，两个SMTP服务器之间通讯是不可能要求登陆、认证、密码之类的东西的，好像是说转发权限被拒绝。难道SINA被163拒绝？