在 mail server的log裡發現

postfix/smtp[1921]: 347A79E0739: to=<abc@gmail.com>, orig_to=<abc@bbb.edu.tw>, relay=alt1.gmail-smtp-in.l.google.com[74.125.25.27]:25, delay=239783, delays=239779/0.04/2.6/0.84, dsn=4.7.0, status=deferred (host alt1.gmail-smtp-in.l.google.com[74.125.25.27] said: 421-4.7.0 [2.3.4.5      15] Our system has detected an unusual rate of 421-4.7.0 unsolicited mail originating from your IP address. To protect our 421-4.7.0 users from spam, mail sent from your IP address has been temporarily 421-4.7.0 rate limited. Please visit 421-4.7.0 http://www.google.com/mail/help/bulk_mail.html to review our Bulk 421 4.7.0 Email Senders Guidelines. d5si4976585pdi.47 - gsmtp (in reply to end of DATA command))

被google擋信了
查了一下google的網頁說明
有三個方式可以處理
最方便的是在DNS設定spf

;spf for google
bbb.edu.tw.  3600  IN   TXT   "v=spf1 ip4:2.3.4.5 include:_spf.google.com ~all"

設定後重啟DNS
等cache更新後就ok了