sending e-mail with php only works this way?

Hello, I'm trying to set up a simple activation e-mail system where a user fill's in a 1 text box form with their e-mail address, on the next page i have it set up to send an e-mail to the address they specified with a url that they can go to to finish off the process, but for some reason the e-mails will not get sent to anyone outside of my domain name???... for example my domain is www.lesserknownzone.com and if me or anyone with a somebody@lesserknownzone.com fills out the form they get the e-mail sent to them no problem and everything works fine. But any other address like somebody@hotmail.com or somebody@mnsi.net they don't get the e-mail sent to them?. I'm wondering if this is a problem with my page or with my webserver?... here is the code on my page: <? $toaddress = "".$email.""; $subject = "Lesser Known Membership Activation"; $mailcontent = "If you are the intended user of the Lesser Known account please Click or copy and paste the URL into your browser to be taken to the membership set-up page: http://www.lesserknownzone.com/inTheStudio/membershipForm.php?username=".$email."\n\n". $fromaddress = "webmaster@lesserknownzone.com"; $header = $fromaddress; mail($toaddress, $subject, $mailcontent, $header, $fromaddress); ?> Like i said everything works fine if the person is using an @lesserknownzone.com e-mail. any help is much appreciated. Thanks, Cory