Knuddi,
Can you further explain/clear up for me, your setup a little bit more? You have 3 offices connected together via VPN. How are you autoforwarding the e-mail to the correct branch office? Do you have all of your users defined on the main branch server, then simply setup their e-mail to forward to a specified email address? Branch offices are setup to send outgoing mail to the main office? If so, did you have to make any other modifications for the main server to accept the mail from the branch servers, other than putting them in the local networks? Finally, did you have to add the other two domains to your etc/hosts file in the main branch so that it would know how to resolve the branches, then the smtproutes so it would know where to send the data?
I hope I'm kind of close.
Thanks,
JB