mail(); works, but only for mails to my (mail)server

Hey you guys! First, I have to say, that php.net gave me a lot of support in the last few weeks and months. It's been great to suck information from your guys' website! Thanx a lot! This weekend, I've been fighting with some really weird problem though... I installed a few mail(); actions on my website plus in some projects I'm working on. I have one really big problem though... Everything works fine until I want to send a mail to another mailserver than mine. So, it only works for my own (mail)server - how the hell is that possible? And: can I fix that? Do I have to include special headers? Thank you very much for yar answer! Greetz Manuel, Berne, Switzerland