I know this is an old thread, but recently had a customer request multiple PTR names for a single IP, and they keep coming back asking, is it done, is it done? Like the kid in the back seat, are we there yet? They keep doing an nslookup on the IP and it keeps coming back with only one name. Well, duh, thats how it works.
To answer this correctly after doing hours of research and finding the correct answer, I thought I would share.
The correct method of doing a PTR record for a mail server with multiple domains using one single IP, IE a hosted mail server, use the servers host name for the PTR record. What you can do to verify my answer, send an email from each domain off that server to an account you have access to and look at the headers. You will find that the received from server name will all be the same, the servers name. When a receiving server looks at an email it will be looking at the headers, and will run a PTR check against the name found in the header, in most case, all the server needs to allow it through is for a name to come back, regardless if it matches or not. For servers that have strict anti-spam filters, it does an actual name match, PTR to server name, and if it isn't and exact match, will reject it. It gets that name from the headers. Check it out, look at the headers of some emails you have, chances are the server name matches the forward lookup for most emails, but you will find a few that don't.