Reverse DNS Setup for an IP with multiple domains

Mail servers will cross-check your SMTP server's advertised HELO hostname against the PTR record for the connecting IP address, and then check that the returned name has an address record matching the connecting IP address. If any of these checks fail, then your outgoing mail may be rejected or marked as spam.

So, you need to set all three consistently: The server's hostname and the name in the PTR record must match, and that name must resolve to the same IP address.

Note that these do not have to be the same as the domain names for which you are sending mail, and it's common that they are not.

It seems like you have misunderstood how PTR records work and what is it for. The question doesn't include any information on how you have tried to set the record:

1. It doesn't change automatically when you chance something.example.com. A record.
2. It can't be set within the example.com. zone but in a reverse zone like 113.0.203.in-addr.arpa.

Like described in my answer on similar question:

It depends on your ISP and your internet connection contract whether you are allowed to request for PTR record changes for your public IP, as the PTR records are managed by the registered owner of the IP block. You can do a lookup on ARIN WHOIS IP Address Database to see the actual owner.

Another answer mentions that you could "overload" PTR records i.e. have many PTR records for a single IP address. That is not recommended because PTR record is expected to identify the canonical name associated with the IP address. (See. RFC 1035, 3.5 and RFC 1912, 2.1.)

You can't have the reverse (PTR) records in the same zone as your forward (A, MX, etc.) records.

They belong to two different DNS zones:

• websitename.co.uk - forward zone. Contains A, MX, CNAME records (like websitename.co.uk 14400 IN A 12.34.55.236)
• 55.34.12.in-addr.arpa - reverse zone, contains PTR records for all IP addresses in the range 12.34.55.x

You obviously control the forward zone, but the reverse zone is controlled by whomever owns the IP addressing range (range 12.34.55.224/28 is allocated to AT&T).

You will only have working reverse resolution once you change the PTR records in the reverse zone, you should get in contact with your hosting provider and request an update to the PTR records.

The good news is that you can have all of your reverse records configured in the same reverse zone, as long as the IP addressing is in the same subnet, for example:

236.55.34.12.in-addr.arpa.  14400 IN PTR websitename.co.uk.
236.55.34.12.in-addr.arpa.  14400 IN PTR websitenameOne.co.uk.
238.55.34.12.in-addr.arpa.  14400 IN PTR websitename.co.uk.
241.55.34.12.in-addr.arpa.  14400 IN PTR websitenameTwo.co.uk.

Note you can "overload" PTR records, i.e. have multiple names for an IP address, the same way you can have multiple IP addresses for a name (A record).