How to configure Exchange 2016\2019 and DNS if the domain name is the same as the site name?

Question

Domain Name System

Lately some domains started returning messages with the error
Remote Server returned '400 4.4.7 Message delayed'
I found a similar case on technet, they advised me to check DNS name resolutions. The site name and email domain name (@domain.ru) are the same.
I did nslookup domain.ru - gave out the IP of the site, which is quite logical. I don't know if this is the problem?
The public DNS has MX, SPF, A records for the mail server. PTR (registered at the provider, points to domain.ru)
I went to https://mxtoolbox.com/ , did diagnostics of the mail server, it gave me
550 5.7.54 SMTP; Unable to relay recipient in non-accepted domain

I don't know for sure if it has something to do with the identical domain and web server names. Exchange set up just recently, he himself has not yet fully figured out the subtleties. Tell me what could be the problem and how to solve it? I would be grateful if you point your finger to the newbie in the right article or just share your experience.
Thanks in advance.
PS: Fixed the PTR record and FQDN in the Send and Receive connectors (default Frontend MXSERVER), but this did not solve the problem with the error Remote Server returned '400 4.4.7 Message delayed' . Letters do not go to some addresses.
PS: The error was not related to the topic of the question, so I'm closing it. Thanks to everyone who found errors in the configuration and responded!

Answer 1

Start with the first mistake. You seem to have a wrong PTR record.
If you have a static IP, contact your ISP to have the PTR for your IP address point to your domain name.
If you have a dynamic IP, then, again, contact your ISP for a white static address.

Answer 2

So what helped in the end?