I want Postfix to differenciate local from external adresses
 

I want my RedHat 7.3 server to act as a email server. The clients are Windows 9X/XP with Outlook 2000. I have 6 email accounts on a ISP (domain mmm.com). The redhat server connects to my ISP and retrieves the email thanks to fetchmail (it works fine). The domain on my LAN is ggg.local. When I send and email to a local user my redhat server (running postfix) delivers it correctly and the from adress of the message is: joseph@ggg.local. The problem is that if a send an email to an external adress (such a albert@hotmail.com) postfix delivers it correctly but the from adress in the message is joseph@ggg.local so Albert can't reply the message (ggg.local is not a public domain).
If I try to use the canonical feature of postfix I can change joseph@ggg.local for joseph@mmm.com (that is an account on my ISP) so albert can reply. The problem is that, in that case, if i send a message to a local user postfix also changes joseph@ggg.local for joseph@mmm.local so when the local user wants to reply the message is not delivered localy but sent to internet.
How can I make, if possible, postfix to differenciate local from external adresses. I want postfix to use canonical only if the TO adress is external from my lan.

Thanks.