Problems with raising the mail server or how to raise it without hemorrhoids?

Question

linux

And hello again, comrades, I was in trouble in the form of raising the mail server. Briefly:
There is a large office with branches, each branch has its own mail server. (Actually, each branch has its own administrator). Everything was fine for the time being, it worked stably for 7 years, but one fine (or not so) day the mail server crashed, without backups and nothing (my cant overlooked that). We set the task to raise the mail server. I'm still quite a novice in this business, but the authorities said - I need to do it.
I decided to raise this matter with little bloodshed, i.e. choose a ready-made assembly for mail and iRedMail got into my eye.
Actually, there were no problems with installing CentOS and iRedMail, everything worked. (here is a remark that each branch has its own "unregistered" domain \all mail is local on the intranet\). Those. for the internal virtual domain mail is configured, clients accept mail and send. (again on the local domain). But when sending mail to other branches (for example, [email protected]), the same RoundCube shows an error: Sender address rejected: Domain not found. Those. I understand that "our" mailer does not see where and to whom to send letters. Actually, then I have 2 questions:
1. How to solve this problem so that the "our" mailer sees other mail servers on the network?
or
2. How to quickly and without hemorrhoids raise a mail server so that mail goes to all "branches" but with "
If there are points in the question that need to be clarified, then I will give an answer. Thanks for the replies in advance.
upd: Please do not kick hard, because. I have been working at the branch for only a month and a half and I try to understand everything according to the manuals, but there is not always a result.

Answer 1

And in DNS all records are present?

Answer 2

You have two options:
1) Manually fill in /etc/postfix/transport with lines like
filial2.firma smtp:[pochta.filial2.firma]
and for each branch mailer in /etc/hosts add a pair of IP ↔ pochta.filial#.firma
2) Everything is almost the same as in the 1st option, only in the postfix config do not write [] for the hostname, but use the local dns server instead of the localhost.