==== Required information ====
- iRedMail version (check /etc/iredmail-release): IredAdmin-PRO 2.5.0
- Linux/BSD distribution name and version: Ubuntu 16.04
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): PGSQL
- Web server (Apache or Nginx): APACHE
- Manage mail accounts with iRedAdmin-Pro? Yes
- Related log if you're reporting an issue:
====

Hi, i'm testing a new server on cloud and i wan't to make a structure where

Internet ----(mail to @mydomain.com)-->( Front-end Server --- EFA ) -----( Back-end Server ---Iredadmin PRO ) --> (deliver to mydomain.com account

Client ( mail from @mydomain.com) ---> (back-end Server named beposta ) ------> (EFA-Server named feposta)------>Internet

If i change parameter relayhost in (BackEnd) postfix_main.cf and set it to SMTP:front-end i cannot send any mail and i cannot receive any mail.
Without this i can receive all mail from Internet but i cannot send

Where i'm wrong ?

Thanks for support

Here is the log

[feposta postfix/smtpd[5071]: NOQUEUE: reject: RCPT from beposta.mydomain.com[X.X.X.X]: 450 4.1.8 <user@mydomain.com>: Sender address rejected: Domain not found; from=<user@mydomain.com> to=<externalaccount@externaildomain.com> proto=ESMTP helo=<beposta.mydomain.com>