PTR record created, but not found

or are they stoerd in separate zones?

Separate zones, one per old C network (last byte in the octet).

What stops me from adding a PTR record saying that resolves to say gmail.com?

Nothing. But as this is not used exceptt for nice pings or some email validity checks, you achieved nothing. people will still go to gmail when they type in gmail.com. All people now see is gmail.com in a traceroute, nothing else.

The one real use for this is smtp - the HELO string given in SMTP should match the PTR record name given. Basically the server must say it is who the ptr record says it is. Note that it can still accept emails for other domains.

To answer your new question first: No, PTRs don't tell you anything about the sender's domain. See below for the explanation.

Now back to your original question:

Receiving mail servers check none, one, many or all of the following:

1. Is the HELO name equal to the hostname (A record)?
2. Is PTR of the IP equal to the A record of the hostname (hostname ==(PTR)==> IP ==(A)==> hostname)?
3. Is the IP part of the provided SPF record?
4. Does the sender's domain have at least one MX record? Which doesn't have to match IP/hostname.

Receiving mail servers that check if the sending server is also the MX server are badly configured and should be eliminated from the Internet.

Edit: The PTR does absolutely prove nothing about the email domain. It is never meant to prove this. There are thousands of domains hosted at Google, Amazon, AOL and others. But none of them match the hostname or PTR of Google, Amazon, AOL and others. They all have the servernames of the providers. And there is nothing bad about that.

The PTR only proves the identity of the server but not the identity of the hosted domains. Point.

2nd Edit: A good example for a working environment would be

• HELO = mail.example.com
• hostname = mail.example.com
• A record of mail.example.com = 172.20.25.25
• PTR of 172.20.25.25 = mail.example.com
• Domains hosted on this server/system = example.com, *.example.com, *.example.net, *.example.org, mycompany.invalid and many many more.
• SPF records of hosted domains (optionally) = v=spf1 a:mail.example.com -all
• MX record of hosted domains can be anything. E.g. mx1.example.com, mx2.example.com, mailfilter.anti-spam-corp.invalid, mail.example.com, postini.google.invalid, ...