sendmail报错dsn:serviceunavailable的解决办法

后来在google上找了很多，最后终于找到原因了：
I fixed the problem by changing the hostname of my server. Apparently "localhost.localdomain" will get rejected automatically by a lot of mail servers. As soon as I changed my hostname to "extra.idahostatesman.com" (a domain that is hosted on the server), everything just worked and I had no more problems sending e-mail.

可以用hostanme 命令临时修改主机名，hostname aa.com; 还是失败了

Aug  1 19:11:33 localhost sendmail[5579]: o71BBVB9005579: from=root, size=221, class=0, nrcpts=1, msgid=<201008011111.o71BBVB9005579@aa.com>, relay=root@localhost
Aug  1 19:11:35 localhost sendmail[5580]: o71BBX3b005580: from=, size=436, class=0, nrcpts=1, msgid=<201008011111.o71BBVB9005579@aa.com>, proto=ESMTP, daemon=MTA, relay=localhost [127.0.0.1]
Aug  1 19:11:35 localhost sendmail[5579]: o71BBVB9005579: to=favorite214@139.com, ctladdr=root (0/0), delay=00:00:04, xdelay=00:00:02, mailer=relay, pri=30221, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (o71BBX3b005580 Message accepted for delivery)
Aug  1 19:11:39 localhost sendmail[5582]: o71BBX3b005580: to=, delay=00:00:04, xdelay=00:00:04, mailer=esmtp, pri=120436, relay=mx.139.com. [221.176.6.96], dsn=4.0.0, stat=Deferred: 450 Requested action not taken:cmmx01, AS0QrJA75wiRVlVMWIvlAA--.3152S2, dynamic, reputation, zen.spamhaus.org, please try again

切记：修改完hostname以后需要重启sendmail 服务，重启服务后，一切ok.