Believe it or not, it can potentially be harmful to setup your e-mail account (@yourdomain.com) to forward all incoming mail to Gmail, America Online (AOL) or any other ISP (Internet service provider). This feature will not only forward all your legitimate incoming mail but also any sp@m that is received in that particular account as well.
The majorities of ISPs offer a “this is sp@m” button to report any unsolicited mail. With only a few reports, your server may banned from sending e-mail to any of their registered users. When these companies receive your forwarded sp@m messages, all that can be determined is that your server sent it!
If you would like to properly access your e-mail through your Gmail account, please read the following article explaining how to “fetch” POP3 e-mails rather than forwarding them: http://mail.google.com/support/bin/answer.py?answer=21289