為何寄到SINA的MAIL總是被退回呢?

我的MAIL是智邦的，MS7.URL.COM.TW，寄到SINAMAIL.COM的郵件總是被退回，
退回的訊息如下：

Connected to 203.73.94.211 but sender was rejected.
Remote host said: 552 Sorry, we don't allow mail from your host.

552的訊息是什麼呢?
已經試了好幾個月都不行，為什麼呢?
有用HINET及東森上網都一樣!!!

Connected to 203.73.94.211 but sender was rejected.
Remote host said: 552 Sorry, we don't allow mail from your host.
照上面的字意
連接到203.73.94.211但是寄件人被退回
遠端主機回復: 522 抱歉,我們不允許郵件來自您的主機
.
看來您用的smtp不被sina接受