公司Exchange 2003邮件服务器使用了Cisco PIX防火墙进行保护之后，如果启用SMTP验证, 客户端发邮件时总是出现错误：Unable to relay for 目标邮件地址，取消SMTP验证则可以正常发送邮件，原以为是Exchnage 2003的问题，可是如何怎么调整Exchange 2003设置，都解决不了这个问题，后来在网上搜索到一篇文章，才知道罪魁祸首是Cisco PIX防火墙，这是Cisco PIX的一个Bug引起的，最新版本的PIX软件已经解决了这个问题。文章中提到的解决方法如下：
Yes, you will want to disable the SMTP fixup. It causes a lot of problems
with exchange. Use the command: "no fixup protocol smtp 25" to disable it.
It acts as a proxy to the smtp server by filtering the commands that it
sends to the server. In doing this, it tends to cause problems with the
sending and receiving server.
There is also a known bug that keeps biting me regarding how the PIX handles
the end of a SMTP message. If the end of message signifier . <CRLF> come in
on seperate packets, the pix doesn't recognize it as the end of the e-mail.
Because of this, the PIX never acknowledges that the message was received
successfully. So, my local SMTP server keeps resending the message to the
client.
There is a documented PIX bug on cisco's site. They have fixed the bug and
it is in the newer releases of PIX software.
Check out the bug if you have a CCO login
<http://www.cisco.com/cgi-bin/Support/Bugtool/onebug.pl?bugid=CSCds90792>