LOCAL SERVERS + SYNCH + ACTIVE DIRECTORY

[Mariano]

Hello to everyone and thank you for the time you will invest in this email.

My company has one main office and 7 other offices in places where the internet connection is unstable and often quite slow (2 Mb/s).
We are using Microsoft Products but we use IBM Lotus for the emails. We have a central server email and 7 little email servers in our offices.
In particular we use IBM Lotus because we configured the 7 little email servers in order to understand when to use internet connection and when is not necessary (the server understands who is a "local user" to the little servers so the email is not sent outside). Practically the main server and the little server synchronize every 5 minutes the "outgoing and incoming emails" (those who use internet).  These servers are accessible just from inside the company (VPNs) and we use a unique domain (e.g. company.com) 
Now my company would like to use webmail so than when people are in business trips they can access always the emails.

I am wondering if is available a solution in linux that can be configured in order to provide the same "trick" so that the "local users emails" are not sent to the other servers so we can reduce the internet traffic. Furthermore it is necessary that the users of the email server log in using Active Directory.

Do you have any idea on how to create such structure? Please help