Which free mail server to use for LAN?

Question

Mail server

Good day. Need advice on which free mail server to use in LAN.
It is necessary that it necessarily have a function for redirecting to ip by domain (as in wingate).
I'll explain exactly what is needed. There are several offices, each has a wingate with its own internal domain.
In wingate, there is a setting to redirect letters from a certain domain to a certain ip on which another wingate is located. This is necessary so that office users exchange mail inside, as well as send letters to other offices.
I can’t make a single server, because if the Internet is turned off in the office with the server, the rest of the offices should be able to work.
Is it possible to do this with some free software?

Answer 1

sendmail?

# cat /etc/mail/mailertable

domain1.ru                       smtp:[10.8.1.5]
domain2.ru                    smtp:[10.7.1.6]

Answer 2

If the problem is internal communication, then wouldn't a locally deployed Redmine be right for you?