I have installed postnuke for my website but the email from postnuke doesnt work. I assume it has something to do with the SMTP Authentication. The mail deliver to any email that belong to my domain (for example, ap2k2@mydomain.com) but not to yahoo, hotmail etc. or any any others.

What happen here?

Hello,
If your running cpanel login to WHM login to it and go to "Tweak Settings", make sure you don't have "Prevent the user 'nobody' from sending out mail to remote addresses (php and cgi scripts generally run as nobody if you are not using phpsuexec and suexec respectively.)" enabled.