> The original message was received at Tue, 25 Mar 2008 10:55:07 +0800
>
>   ----- The following addresses had permanent fatal errors -----
> <[email=A@X]A@X[/email]>
> <[email=B@X]B@X[/email]>
>
>   ----- Transcript of session follows -----
> .. while talking to sbcmx9.prodigy.net
> >>> MAIL From:<[email=C@Y]C@Y[/email]
> <<< 553 5.3.0 nlpi049,DNSBL:521< 210.51.23.8 >_is_blocked.__For_information_see:http://www.att.net/bls_rbl/
>

这是什么意思呢？DNSBL是黑名单啊，那么521< 210.51.23.8 >_is_blocked呢？

553
sorry， that domain isn't in my list of allowed rcpthosts
mailbox name not allowed
邮件地址错误
Requested action not taken： mailbox name not allowed
sorry, your envelop sender is in my badmailfrom list
所要求的动作未执行：信箱不允许该动作执行 ，或者服务器限制了收件人的地址，只能换一个smtp服务器发信。

Relay restriction.
From <>, message blocked.
sorry,you are not allow to use this SMTP to relay your eami
sorry, that domain isn't in my list of allowed rcpthosts
使用21cn.com，china.com，371.net，sina.com等大多数信箱的smtp服务器时，只能用自身的信箱发信，所以要在Outlook Express的"帐户属性"中的"个人信息"里面填写正确的邮件地址。

authentication is required to send mail as <>
使用263.net和sohu.com的Smtp服务器时，不但要用自身的邮箱发信，而且要加入身份验证，所以即要在"个人信息"中填写正确邮箱地址，又要选中"SMTP服务器需要认证"。

<>...domain name required
多出现在用Wingate代理服务器发送邮件时。虽然在Wingate中"POP3 邮箱帐号"要使用"用户名#POP3地址"的格式，但在"帐户属性"中的"个人信息"中还是要填写一般的格式。

521
This system is configured to reject mail from X.X.X.X
安全因素，被封锁

或換一個dns