mail not sent to certain mail accounts

Hi all. I'm using the PHP mail function to send a confirmation email to a person once they register on the site. A few thing that happens with 2 of the mail accounts that we're testing to receive the mail is that the mail never is received. One of the mail accounts is a Yahoo account, the other a local email from a service provider. What could be the factors contributing to this? Could it be anti-spam software? or could there be something else that is blocking the mail? Or could there be a problem with my PHP mail function? All other mail account types (eg: hotmail, gmail etc...) are able to receive the email. Any help would be great. Thanks in advance! -- Angelo Zanetti Z Logic www.zlogic.co.za [c] +27 72 441 3355 [t] +27 21 469 1052