It looks like Yahoo and Hotmail are now blocking emails from this webserver. Whether or not AOL still is, I don't know.
This means that if any new users try to sign up from a Hotmail or Yahoo address, they will never receive their activation emails.
This has been a problem for a while - see
http://superman.nu/smf/index.php?topic=2053.0 for the AOL problems.
New users are still advised to use a more friendly email service; or to contact me if you'd like your account manually activated. Should you choose the latter, however, you will still be unable to receive any password reminder emails, thread-update notices, etc.