==== Required information ====
- iRedMail version (check /etc/iredmail-release):     v0.9.5-1
- Linux/BSD distribution name and version: CentOS 6.8
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): LDAP
- Web server (Apache or Nginx): Nginx
- Manage mail accounts with iRedAdmin-Pro? Yes
- Related log if you're reporting an issue:
====

In the main.cf file for Postfix a note is made:

# trusted SMTP clients which are allowed to relay mail through Postfix.
#
# Note: additional IP addresses/networks listed in mynetworks should be listed
#       in iRedAPD setting 'MYNETWORKS' too. for example:
#
#       MYNETWORKS = ['xx.xx.xx.xx', 'xx.xx.xx.0/24', ...]

Where is this configuration file I am to update?

Thanks!!