What should Reverse DNS be set to?

Operating system: CentOS
OS version: 8

I have virtualmin on a VPS. I initially named it srv
So with my domain, its srv.domain.com.
Setting up virtualmin, it looks like the email is linked to mail.domain.com.
What should I set my reverse DNS to?
domain.com
srv.domain.com
mail.domain.com
should I change my hostname to mail?

I have other domains pointing to my server too
mail.domain2.com
mail.domain3.com

everything points to my one IP

I can only set my IP’s reverse DNS to one thing

It doesn’t matter as long as it resolves in both directions (i.e. whatever name it resolves to should resolve back to the same IP). The PTR can be domain.tld, but if you’re hosting mail for domain.tld you should not change the hostname to that.

You should not change your hostname to mail in any domain you host on the server (and hostname does not have to match reverse, but can as long as it isn’t a name you’re hosting virtual mail for).

Ok. Was trying some online mail server checks and it complained that the IP didn’t resolve to mail.domain.com. I want sure if it would affect SPAM filters. Note my DNS is on cloud flare

As I said, reverse can be mail.domain.tld, if you want it to. But, your system hostname should not be.

Edit: But also those checkers are wrong, if they actually say what I think you say they’re saying. As long as you get DKIM, etc. right, the PTR just needs to resolve, it does not need to match some arbitrary name within one of your domains (it obviously cannot match all of them, since PTR is for the IP).

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