All Emails from the forums failing

[nick_1]

OK, basically the problem is this, trying to send any type of e-mail from the forums fails. This includes the system sending out e-mail notification or even using the e-mail forms.
I'm concerned that it might be the Zend optimiser as thats a recent addition.

Here are the server details...

URL: forums.portent.net
PHP Version: 4.0.4 with zend optimiser
Mysql version: 3.23.22-beta
Apache/webserver version: Apache/1.3.19
Server operating system: Redhat 7.0