I have a user with 8 companies so ... domains & I have set these domains up

Different users work as different companies
Some users work as more than 1 company so I setup accounts for these with Proxy access

but, a couple of users work for 2-3 companies and want the emails coming to their single mail box
I can display the 'to' field in GW client so they know which domain
however, they want to SEND-AS the domain based on incoming domain name

Whats the best way to do this (with separate account and proxy its not a problem!)