I am using iRedMail on Ubuntu 10.04.3 with iRedAdmin v0.1.6 (MySQL).

I get this message when I send to a user on the same server, in the same domain:

<<This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

  tracydow@dowelectric.com
    SMTP error from remote mail server after RCPT TO:<tracydow@dowelectric.com>:
    host mail.dowelectric.com [24.213.188.74]: 553 5.7.1 <rick@dowelectric.com>:
    Sender address rejected: not logged in

------ This is a copy of the message, including all the headers. ------

Return-path: <rick@dowelectric.com>
Received: from reseller9.hrwebservices.net ([66.147.230.20])
    by relay2.hrnoc.net with esmtp (Exim 4.63)
    (envelope-from <rick@dowelectric.com>)
    id 1S4eOM-0003DC-2V
    for tracydow@dowelectric.com; Mon, 05 Mar 2012 15:20:27 -0500
Received: from cpe-69-205-16-139.nycap.res.rr.com ([69.205.16.139] helo=[10.1.1.4])
    by reseller9.hrwebservices.net with esmtpa (Exim 4.69)
    (envelope-from <rick@dowelectric.com>)
    id 1S4eOM-00034I-T4
    for tracydow@dowelectric.com; Mon, 05 Mar 2012 15:20:22 -0500
Message-ID: <4F551FFB.4010305@dowelectric.com>
Date: Mon, 05 Mar 2012 15:20:11 -0500
From: Rick Conlee <rick@dowelectric.com>
User-Agent: Thunderbird 2.0.0.24 (Windows/20100228)
MIME-Version: 1.0
To: Tracy Dow <tracydow@dowelectric.com>
Subject: test again
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
X-HR-Scan-Signature: 90ac36c35b3b451865a0ad381c7bdc61
X-HR-ClamAV-Scan: Clear
X-HR-SA-Score: 0.0 (/)
X-HR-Status: Normal-(rick@dowelectric.com/66.147.230.20)

Still testing... Get the mobile phone going?

-Rick
>>

Users have been calling to complain of similar issues. I looked around the forum and have yet to find a satisfactory solution.

The email server hostname is the same as the DNS name - and is resolvable. The issues seem to happen to users inside rather than users outside. 

==== Provide required information to help troubleshoot and get quick answer ====
- Linux/BSD distribution name and version:
- iRedMail version and backend (LDAP/MySQL):
- Any related log? Log is helpful for troubleshooting.
====