Please ALWAYS show us basic info of your iRedMail server, so that we can give you quicker answer:
==== Required information ====
- iRedMail version:
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):
- Linux/BSD distribution name and version:
- Related log if you're reporting an issue:
====
iRedMail supports per-user and per-domain transport. for OpenLDAP, it's defined in attribute 'mtaTransport', for SQL backends, it's defined in column "domain.transport" (per-domain) or "mailbox.transport" (per-user).