I help a friend run a server, which includes a mail server. http://mxtoolbox.com reports that our reverse DNS is not a valid hostname.

The reverse DNS currently points to domain.com. Should it point to hostname.domain.com? I have seen here that it is conventional to use this scheme for reverse DNS. However, will this cause a problem if the mail server responds like this:

220 domain.com ESMTP Postfix (Ubuntu)
EHLO PWS3.mxtoolbox.com
250-SIZE 10240000

Basically, will the reverse DNS pointing to a subdomain of the domain the mail server reports be valid?

EDIT: My original question was as follows:
If the mail server reports domain.com as its FQDN, can the reverse DNS point to hostname.domain.com? See comments below for why this isn’t needed, and why both should be the same.

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