mail() problem

Hi, i am currently having some problems with the mail() function, because emails don't get send, or if they do, they arrive with a huge delay. (~ 5% of mail() sent emails arrive) This only happens, if i try to email s.o. outside my network. Does anyone have an idea, what might cause this problem? Could it be the ISP, or do i need to add additional info, when i call the mail() function in order to send emails outside my network? This is what i used to test if the emails get sent or not: <? mail(test@test.com,"test-subject","Test-content","From: test@test.com\r\n"."Reply-To:test@test.com\r\n"); ?> Regards, Duncan