One ip + many mail domains + one ptr = ???

Question

Domain Name System

I have a lot of websites on vps / vds . Accordingly, one ip and many mail domains. I can only point the PTR to one mail server . Let's say I have a domain test.com. I indicated it in the FQDN of the VPS, and in the PTR I indicated mail.test.com
Please write what dns records I need to make for domains located on this vps.
I know that you need to make an MX record, specify "mail.test.com." in the host, and the ip address in the value. Correctly?
What other entries need to be made?

Answer 1

The PTR record does not have to match the domain, the main thing is that it matches when checked by the receiving server, when determined by IP and what is in the message headers. There can theoretically be an unlimited number of domains on one IP, this is how regular shared hosting works.
Configure DNS as usual, MX, SPF, DKIM for each domain that works on your server

Answer 2

In such a situation, you need to pay attention to the mail.test.com domain zone -> you must specify SPF for it, otherwise recipients may lose Bounce-messages (NDR) from your mailer, because 5321.mailfrom for NDR messages is usually empty - then the receiving server will check the SPF by name in the HELO/EHLO SMTP banner (in your case, mail.test.com).
Everything else - everything is as usual for any mail domain.

Answer 3

I can specify PTR to only one mail server.

Wrong.
You can specify multiple PTRs per 1 ip. It is the responsibility (should) of the provider to enter domain names in the reverse zone.
Inability to enter multiple PTRs - The provider did not provide or limit the number of PTRs. Call write a letter who gave you the ip address with a request to register return addresses.