Change mail attachment size

==== Required information ====
- iRedMail version (check /etc/iredmail-release):0.9.4
- Linux/BSD distribution name and version: Debian 8.2
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):MySQL
- Web server (Apache or Nginx):Apache
- Manage mail accounts with iRedAdmin-Pro? No
- Related log if you're reporting an issue:
====
Hi.
I executed 2 commands:
# postconf -e message_size_limit='20971520'
# postconf -e mailbox_size_limit='104857600'
but all the same there don't pass letters from Outlook of 18-19 MB, it is only less than 15 MB.
If to execute - postconf - n message_size_limit,  that removes 20971520