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<taems@mail3-79.sinamail.sina.com.cn> (expanded from <taems@sina.com>): host
mail3-79i.sinamail.sina.com.cn[10.55.2.79] said: 521 5.2.2 can not find the
user (in reply to RCPT TO command)

这是你发E-mail失败，系统退信的一个回复。因为我经常收到。
具体什么意思我不翻了，我告诉你发生的原因及解决的办法

1。对方E－MAIL地址错-->改为正确的地址
2. 对方因为受到垃圾攻击，而进行了邮件过滤，正好你的IP或者用的邮件在对方过滤的范围。-->致电对方，请对方的IT部门解决
3.对方信箱满了-->请对方清理一下邮箱。
4.对方的邮箱所在网站在做系统维护-->过些时候等维护结束再重新发

呵呵，基本上我遇到过的就这么些的原因。