==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 0.9.9 MySQL
- Deployed with iRedMail Easy or the downloadable installer? - Downloadable
- Linux/BSD distribution name and version:  Ubuntu 18.04 LTS
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Web server (Apache or Nginx): nginx
- Manage mail accounts with iRedAdmin-Pro? No
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

I have a mailbox test@test.com set up with iRedmail that sends mails via external SMTP relay. It works fine when I send an email from that mailbox.

I set up forwarding to forward all mail from test@test.com to something@gmail.com. When I send mail to test@test.com, the message is not forwarded to the final destination. However, the logs show that postfix connected to the external SMTP:

Oct 26 16:26:51 mx postfix/smtp[32209]: Anonymous TLS connection established to smtp.postmarkapp.com[52.208.184.61]:587: TLSv1.2 with cipher ADH-AES256-GCM-SHA384 (256/256 bits)
Oct 26 16:26:51 mx postfix/smtp[32209]: 470mZy6nwwz2wJY: to=<millersergey777@gmail.com>, relay=smtp.postmarkapp.com[52.208.184.61]:587, delay=0.53, delays=0.01/0.02/0.45/0.05, dsn=2.0.0, status=sent (250 2.0.0 Ok: queued as 6D4A0413061)
Oct 26 16:26:51 mx postfix/qmgr[32089]: 470mZy6nwwz2wJY: removed