I recently did a mailing to a lot of hotmail addresses and now all the e-mails I send out (from PHP by the way) don't seem to reach their destination. So I figure hotmail banned my url...
Is there any way to get rid of this ban so my e-mails will reach the users again (like activation e-mails etc)?
If your server IP is blocked by Hotmail, try using an alternate SMTP to send the messages temporarily. Maybe Gmail's SMTP? if you have a Gmail account of course. Don't abuse it though.
I am almost 100% sure that Hotmail will remove your IP eventually from the blacklist. Most services run a 48-72 hours ban; if they don't receive any more requests from the banned IP, it will get removed from the list.