Outlook SMTP Send Mail Problem : Client host rejected: cannot find your hostname

recently we faced a problem in Outlook Client Send mail process,sending message will fail with this error :

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Client host rejected: cannot find your hostnamei did rebuild mail server, restarting All mail services and Update ClamAV.. rebooting server and...So many times with no luck.
Here is the last line of MailLog :

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Aug 10 14:02:21 serv postfix/smtpd[13894]: connect from localhost[::1]
Aug 10 14:02:21 serv postfix/smtpd[13894]: NOQUEUE: reject: RCPT from localhost[::1]: 450 4.7.1 : Recipient address rejected: Access denied; from= to= proto=ESMTP helo=
Aug 10 14:02:21 serv postfix/smtpd[13894]: disconnect from localhost[::1] ehlo=1 mail=1 rcpt=0/1 quit=1 commands=3/4
Aug 10 14:02:21 serv postfix/smtpd[13894]: connect from localhost[::1]
Aug 10 14:02:21 serv postfix/smtpd[13894]: NOQUEUE: reject: RCPT from localhost[::1]: 450 4.7.1 <51vero@live.fr>: Recipient address rejected: Access denied; from= to=<51vero@live.fr> proto=ESMTP helo=
Aug 10 14:02:21 serv postfix/smtpd[13894]: disconnect from localhost[::1] ehlo=1 mail=1 rcpt=0/1 quit=1 commands=3/4
Please Help us With this issue.

Hello I have the same problem as you, have you managed to solve it?

Aug 30 03:17:28 host postfix/smtpd[7383]: NOQUEUE: reject: RCPT from unknown[2.139.215.26]: 450 4.7.25 Client host rejected: cannot find your hostname, [2.139.215.26]; from=<Facturacion@susasesores.com> to=<repamagrisl@repamagrisl.es> proto=ESMTP helo=<mail.susasesores.com>

Hello I have the same problem as you, have you managed to solve it?

Aug 30 03:17:28 host postfix/smtpd[7383]: NOQUEUE: reject: RCPT from unknown[2.139.215.26]: 450 4.7.25 Client host rejected: cannot find your hostname, [2.139.215.26]; from=<Facturacion@susasesores.com> to=<repamagrisl@repamagrisl.es> proto=ESMTP helo=<mail.susasesores.com>

I checked and your IP does resolve externally but you have a problem: it resolves to your domain not your hostname! Should be something like server.susasesores.com

Check /etc/hosts has an entry

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2.139.215.26    server.susasesores.com
Change your hostname in CWP admin to server.susasesores.com

At servidoresdns.net
Add a A record for server.susasesores.com
Add a CNAME record for mail.susasesores.com

Use leafdns.com to check when the DNS has propagated for both of the above.
Plus check for updates at http://leafdns.com/index.cgi?testid=6CD93413  (Redo Test)

(Unfortunate name for your secondary mail server: mtmx01.arsenet.com! :-o)

Thanks for answering, the domain susasesores.com is not my property.
The problem is that my client is on my server and can send mail to the domain susasesores.com.
But from the domain susasesores.com when they send my client they don't arrive and he gets an email with the following error.


The delivery to these recipients or groups could not be made:

Repamagri S.L. (repamagrisl@repamagrisl.es)
The server has tried to send this message without success and has stopped trying. Send the message again and if the problem continues, contact your organization's technical support department.



Diagnostic information for administrators:

Generating server: LAMBDA.susasesores.com

repamagrisl@repamagrisl.es
#4.4.7 QUEUE.Expired; message expired ##

Original message headers:

Received: from LAMBDA.susasesores.com ([::1]) by Lambda.susasesores.com
 ([::1]) with mapi id 14.02.0247.003; Wed, 19 Aug 2020 15:52:42 +0100
From: Candelaria Hernandez <CHernandez@susasesores.com>
To: "Carlos Miguel (Byte Canarias)" <info@bytecanarias.com>, Repamagri S.L.
   <repamagrisl@repamagrisl.es>
CC: "Repamagri, S.L. Office" <repamagrisl2@gmail.com>
Subject: RE: Problems with mail
Thread-Topic: Problems with mail
Thread-Index: AdZwmTQamOlJ2R8qRiin3KZu68lkPgBrI1fQAPenswAABP0CMA==
Disposition-Notification-To: Candelaria Hernandez <CHernandez@susasesores.com>
Return-Receipt-To: <CHernandez@susasesores.com>
Date: Wed, 19 Aug 2020 14:52:41 +0000
Message-ID: <0D6F8015623EE541BCF9F6ABE1D260F3029F750ADC@Lambda.susasesores.com>
References: <!&!AAAAAAAAAAAAAAAAJXSTtleURZKopH+P5I4scXCgAAAEAAAAGAveJMcyzVJu5iqmFRm/4cBAAAAAA==@repamagrisl.es>
 <0D6F8015623EE541BCF9F6ABE1D260F3029F74D5FD@Lambda.susasesores.com>
 <ad1968b2-f04a-6730-69ed-f321ee2c063d@bytecanarias.com>
In-Reply-To: <ad1968b2-f04a-6730-69ed-f321ee2c063d@bytecanarias.com>
Accept-Language: en-ES, en-US
Content-Language: en-EN
X-MS-Has-Attach: yes
X-MS-TNEF-Correlator:
x-originating-ip: [172.26.0.193]
Content-Type: multipart/related;
   boundary="_007_0D6F8015623EE541BCF9F6ABE1D260F3029F750ADCLambdasusases_";
   type="multipart/alternative"
MIME-Version: 1.0


Reporting-MTA: dns; LAMBDA.susasesores.com

Final-recipient: RFC822; repamagrisl@repamagrisl.es
Action: failed
Status: 4.4.7
X-Supplementary-Info: < #4.4.7 smtp;550 4.4.7 QUEUE.Expired; message expired>
X-Display-Name: Repamagri S.L.

That last error message series isn't helpful.
You need to get your client to fix their domain.

Client host rejected: cannot find your hostname, [2.139.215.26];

This is a fundamental and serious problem. Without resolving this first, you be applying sticky plasters to work around the problem.
A hostname is not the same as a domain name.

That last error message series isn't helpful.
You need to get your client to fix their domain.
Quote
Client host rejected: cannot find your hostname, [2.139.215.26];
This is a fundamental and serious problem. Without resolving this first, you be applying sticky plasters to work around the problem.
A hostname is not the same as a domain name.