Send mail with site mail-address plz!

Hello everyone, I am running a service desk with multiply email addresses one for every site, I notice that ME SD+ didn't support having multiply email fetching addresses so I made all the other email addresses forward the mail to the one I use for ME.

Now I notice a problem that is that ME SD+ only seems to answer with the mail-address defined in the fetchmail settings and this have become a big issues for us due to our customers may only allow some addresses to pass throw there firewalls. So I want to know if I can make ME SD+ to use the SITE email address as the sender address? Or is there another way doing this, for example faking email headers etc.

Thx!