[BlueOnyx:11864] Can't send mail from PHP without a mailalias

[Dogsbody]

Hi All,

I have two sites (example.com and example.net) We'd like mail to go to 
both domains but separate websites...

Site: www.example.com
   Web Alias - example.com
   Mail Alias - example.com, example.net, www.example.net

Site: www.example.net
   Web Alias - example.net
   Mail Alias - <blank>

... email is running fine and the sites work fine but I can't send any 
email from the www.example.net site at all.

The site is wordpress and it's just using phpmail so I'm guessing there 
is some form of protection included somewhere?

Any guides as to what it is an how to turn if off please?

Thank you :-)

Dan