问了一个同事才知道，MX的优先级别不能设成一样的（默认是10）
* Priority *
* Mail Server *
1
* This is the top priority mail server. Do not assign this priority to any
other servers. Learn
more<https://www.google.com/support/hosted/bin/answer.py?answer=34181>
. ASPMX.L.GOOGLE.COM. 5 ALT1.ASPMX.L.GOOGLE.COM.
5 ALT2.ASPMX.L.GOOGLE.COM.
10 ASPMX2.GOOGLEMAIL.COM.
10 ASPMX3.GOOGLEMAIL.COM.
10 ASPMX4.GOOGLEMAIL.COM.
10 ASPMX5.GOOGLEMAIL.COM.