==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release):  1.0 MARIADB edition.
- Deployed with iRedMail Easy or the downloadable installer? downloadble
- Linux/BSD distribution name and version:  Linux Ubuntu
- 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 am trying to setup a backup server for SENDING OUT ONLY.
Emails from mydomain.com must be sent out to the primary server instead of delivered locally.
As per my understanding for that I need to activate backupmx field and set a relay transport for mydomain.com.

For that I activated backupmx and set the transport with:

UPDATE domain SET transport='relay:[primary.mydomain.com]:25',backupmx=1 WHERE domain='mydomain.com';

Now when I try to send an email I get:
postfix/submission/smtpd[12475]: warning: unknown[10.33.33.4]: SASL LOGIN authentication failed:

And it keeps asking my for the password.
Disabling backupmx with 0, I am able to send emails to external domains. Non-existing users fail with the "Recipient address rejected: User unknown in virtual mailbox table;".

What am I missing?