==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 1.0
- Deployed with iRedMail Easy or the downloadable installer? dl
- Linux/BSD distribution name and version: deb10
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): my
- Web server (Apache or Nginx):
- Manage mail accounts with iRedAdmin-Pro?
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

just a minor bug in rc config.inc.php:
$config['max_message_size'] = '15';
should be:
$config['max_message_size'] = '15M';