spam 跟 域名 跟 Exchange DNS

暨上次小弟整合雙域名的問題後

目前最不了解的地方在於
DNS...

在我的理解中
Internet -> Gateway -(NAT)> SPAM ->Exchange
可是這樣不論a.com.tw或b.com.tw都應該要指向GATEWAY的IP對吧?

目前我用telnet a.com.tw 25 是正常(舊有的)
telnet b.com.tw 25 也可以使用
不過那歡迎訊息是寫 a.com.tw

然後用Gmail寄信做測試
寄出後就會收到
Delivery Status Notification (Failure)
Delivery to the following recipient failed permanently:

et54987@b.com.tw

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain b.com.tw by IP

The error that the other server returned was:
550 M.5 <et54987@b.com.tw>... User unknown(Local Mailbox)

可是我在內部測試的時候，確定有這個信箱啊!~
還請各位前輩指點迷津!!!