Legitimate question - allowing mail from my site

Hi guys,
It seems that yahoo and hotmail (well mostly yahoo) are flagging any mail coming from my site as spam and therefore either blocking it or sending it directly to the trash folder.

This is causing issues with my site as members must activate their accounts via email.

How do I go about contacting yahoo and hotmail and having them add my domain as a legitimate domain?

Thanks,
RJ

Iv had this problem before. I find the best thing to do is have a set Subject for all your messages sent to your account, then in options in Hotmail make a filter accepting any email with this subject.