What is the problem with not delivering messages by the IRedMail mail server?

Question

postfix

There is a corporate mail based on IRedMail. When sending messages to some email addresses, the server swears and returns an error.
This is the mail system at host mydomen.ru.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system

: host mxru[*.2*.*.35] said: 450 4.7.1 Client host
rejected: cannot find your hostname, [*.*.*.233] (in reply to RCPT TO
command)
Reporting- MTA:dns; mypostserver.com
X-Postfix-Queue-ID: 0888D1BF27E
X-Postfix-Sender: rfc822; ***@mydomen.ru
Arrival-Date: Wed, 9 Jun 2021 10:17:48 +0500 (YEKT)

Final-Recipient: rfc822; *@x.ru
Original-Recipient: rfc822;*@x.ru
Action: failed
Status: 4.7.1
Remote-MTA: dns; mxru
Diagnostic-Code: smtp; 450 4.7.1 Client host rejected: cannot find your
hostname, [*.*.*.233]

Answer 1

Is the hostname specified in the mailer settings and on behalf of which mail is sent correct and existing? This recipient (I think, not only him) has checks that your letter does not pass.
Test on www.mail-tester.com and it will become clear where the trouble is and what needs to be tweaked.