==== Required information ====
- iRedMail version (check /etc/iredmail-release): 0.9.4
- Linux/BSD distribution name and version: Ubuntu 14.04
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Web server (Apache or Nginx): Apache
- Manage mail accounts with iRedAdmin-Pro? Yes
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

One of our hosted customers uses a hosted application to send out mass emails.  The 'FROM' address is from one of the domains hosted in iRedMail.  Everything works fine unless one of the destination addresses is also something hosted on the iRedMail server.

For example:

mydomain.com is hosted on iRedMail
They use a 'FROM' address on that domain - user1@mydomain.com
Everything works great, unless one of the people they send to is also a mailbox on mydomain.com - let's say user2@mydomain.com

The error they get is the following:

553 5.7.1 <user1@mydomain.com>: Sender address rejected: not logged in
(in reply to RCPT TO command)

X-Postfix-Queue-ID: F1929120578
X-Postfix-Sender: rfc822; user1@mydomain.com

Final-Recipient: rfc822; user2@mydomain.com
Action: failed
Status: 5.7.1
Remote-MTA: dns; mx01.mydomain.com
Diagnostic-Code: smtp; 553 5.7.1 <user1@mydomain.com>: Sender address rejected: not logged in

Any suggestions on how I can fix this issue?  I've added the IP of the mass-mailing server to the 'mynetworks' section in /etc/postfix/main.cf but honestly, beyond that I'm at a loss on how I can proceed.  Any help would be appreciated.

Thanks.