==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release):  1.4
- Deployed with iRedMail Easy or the downloadable installer? yes
- Linux/BSD distribution name and version:  Rocky Linux 8
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):  MySQL
- Web server (Apache or Nginx): Apache
- Manage mail accounts with iRedAdmin-Pro? no
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====

I have setup iRedMail. It generally works for sending email, but some accounts are not receiving emails. It is kind of a weird mix. I have been able to get the following error from the sending side. I am not seeing anything in the maillog on my side. Any help is appreciated.

Delivery has failed to these recipients or groups:
support@carmonnetwork.com (support@carmonnetwork.com)
The email address you entered couldn't be found. Please check the recipient's email address and try to resend the message. If the problem continues, please contact your helpdesk.

Diagnostic information for administrators:
Generating server: MB04-C.hosting.local
support@carmonnetwork.com
Remote Server returned '550 5.1.1 RESOLVER.ADR.RecipNotFound; not found'
Original message headers:

Received: from MB01-C.hosting.local (98.8.155.63) by MB04-C.hosting.local
(98.8.155.66) with Microsoft SMTP Server (TLS) id 15.0.1293.2; Thu, 16 Sep
2021 21:44:38 -0400
Received: from MB01-C.hosting.local ([fe80::44e5:2c9b:2b30:9a2e]) by
MB01-C.hosting.local ([fe80::44e5:2c9b:2b30:9a2e%17]) with mapi id
15.00.1293.006; Thu, 16 Sep 2021 21:44:38 -0400
Content-Type: application/ms-tnef; name="winmail.dat"
Content-Transfer-Encoding: binary
From: it <email@domain.org>
To: "support@carmonnetwork.com" <support@carmonnetwork.com>
Subject: testing
Thread-Topic: testing
Thread-Index: AQHXq2WToujjbXyXqEKDyHeSCaRPCQ==
Date: Thu, 16 Sep 2021 21:44:38 -0400
Message-ID: <5c959467098c4e9fa01ec2772f6212a1@MB01-C.hosting.local>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach:
X-MS-TNEF-Correlator: <5c959467098c4e9fa01ec2772f6212a1@MB01-C.hosting.local>
MIME-Version: 1.0
X-MS-Exchange-Transport-FromEntityHeader: Hosted
X-Originating-IP: [47.50.100.146]
Return-Path: email@domain.org