Hi there, first of all, thanks for a great product

I have set up 2 YaBB based forums now, one of each type (perl and php). The perl one is running on a webhost, and is working fine.
The php one is set up on my machine here, and I have given it the same settings as the perl one, but it doesn't seem to want to send email. Its version 1.0.0 which I downloaded yesterday.
At the moment, the configuration is :
$mailprog = "/usr/sbin/sendmail";
$smtp_server = "x.x.x.x";
$webmaster_email = "[email protected]";
$mailtype = 1;
Since I read in another thread that changing to the IP address of the mail server could help cure a problem.
Its running on a linux platform, and there is a firewall, however that isn't the problem, since mail clients on the inside can send to the outside world.
Is there something which I am missing?
Thanks for any help!