User can't receive registration confirmation

The web site I’m working on, using a php script, allows users to register and receive an email to confirm their registration.

These confirmations are received to users with yahoo and google email accounts, but these confirmation emails, sent from my domain, are not received to user accounts that have hotmail accounts.

I have looked at these hotmail accounts, in the junk folders, I have added my site’s domain to the hotmail account safe sender list, and checked the hotmail filters and blocked sender list.

Any ideas/remedies/suggestions would be greatly appreciated.

Watch your mail log while you send a mail to a hotmail address. You can even do it yourself over telnet if you want to see the response yourself (SMTP protocol isn’t too hard to type out). Likely you’ll see the reason Hotmail is rejecting mails from your server in its response.

If its sending email to Google and Yahoo accounts, then it is probably not a PHP issue. Like Dan has said check the logs, maybe your domain is barred or something.