Show username for "SASL LOGIN authentication failed:"?

==== 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:
======== Required information ====
- iRedMail version:
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Linux/BSD distribution name and version: Centos 5.x
- Related log if you're reporting an issue:
====

Hi, when an user inputs an incorrect password, I have the following message in the logs:

mx1 postfix/smtpd[1069]: warning: unknown[89.xx.xx.xx]: SASL LOGIN authentication failed: UGFzc3dvcmQ6

Which is perfectly normal.

But how can I also show the username in the logs? I want to see which user keeps entering the wrong password...

Thanks!