Two domain names

I have a server on an intranet whish runs fetchmail to get mail from my
ISP.

Intranet domain mail.domain.com
My ISP domain name domain.com

If some one send mail on the internet they will send it to
user(_at_)domain(_dot_)com but fetchmail cannot foward it to 
user(_at_)mail(_dot_)domain(_dot_)com(_dot_)

Regards Philip Immoos