email server - PTR record (rDNS) for multiple domains on a shared IP address

I host multiple domains on a single IP address:

cats.com 
A @ --> 10.20.30.40
A mail --> 10.20.30.40
MX @ --> mail


dogs.com
A @ --> 10.20.30.40
A mail --> 10.20.30.40
MX @ --> mail

I need to set up a PTR record for the IP address. This is so that forward-confirmed reverse DNS will succeed for other mail servers.

DNS query type PTR on 10.20.30.40 --> returns PTR-record="cats.com" (1 result)

DNS query type A on "cats.com" --> returns A-record=10.20.30.40 (1 result)

If I can only have one PTR record for my IP, that points to cats.com, how will FCrDNS ever succeed for dogs.com?

Can I just use the domain name they both share (80.70.60.50.static.host.net) in the PTR ?

Thanks!

Answer

Reverse DNS can only really have one name -- if you list more than one, DNS will just round-robin between them. So you just have to pick one "official" name for the server and always use that. All the other domains are, in essence, just aliases to the original.

Most services don't care what the name is that your server uses, they just care that the reverse lookup returns a domain name that resolve correctly going the other way as well.