thanks Zhang but i didn't get the point in your answer "If you can sync mail accounts from ISP to your iRedMail server, you can set per-user transport for each user, then emails will go to another mail server for that office"
The domain mails are in the ISP and i would like to have one iredmail server per office with the user's mailboxes that belongs to that office/server. Allow me to explain it different:
Domain mail is example.com that is hosted in the ISP servers.
Office 1 has user1@example.com and user2@example.com
Office 2 has user11@example.com and user12@example.com
I would like to have one iredmail server on each office. The problem i'm having is when user1@example.com tries to send an email to user11@example.com of course postfix block it. My question is: Is there any way to configure postfix or iredmail to support my deployment?
Postfix will check whether recipient domain is hosted locally, if yes, mails sent to non-existing users will not be delivered (user unknown).
If you can sync mail accounts from ISP to your iRedMail server, you can set per-user transport for each user, then emails will go to another mail server for that office.