Have your users set their return address to machinename.yourdomain.com, where "machinename" is the name of the server you've built at the branch office.
Then create a cname record for that machine in the nameserver that hosts your corporate domain nameservice.
bob@mydomain.com and bob@branch1.mydomain.com become two different email addresses. Make sure you update your hostnames with the corporate mailserver so that so that mail sent to your corporate office knows where to go.
This is quite common with companies that have multiple mail servers.
-jeff