CloudFlare and PTR records for reverse DNS

• You can have as many A RRs as you want point to 1.2.3.4.
• You must have one PTR point back to a name.
• Therefore all you need is to make sure that the PTR points to the name you want and that this name points back to 1.2.3.4.

This means that you can have:

example.com. IN A 1.2.3.4

example.com. IN MX 10 mail.example.com.

server.example.com. IN A 1.2.3.4

www.example.com. IN A 1.2.3.4

mail.example.com. IN A 1.2.3.4

and:

4.3.2.1.in-addr.arpa. IN PTR server.example.com.

Why is this the requirement? Because when your SMTP server connects to a remote SMTP server to deliver mail the SMTP server knows only the address of your server and not the name. With the address at hand (1.2.3.4) it queries the DNS and gets the PTR response (server.example.com). The remote server now will ask the DNS again "what is the address of server.example.com" and it expects that answer to be 1.2.3.4.

What you send in the HELO string should not frighten you. You can read about the purpose of the SMTP HELO and find about the exceptions where it is allowed to block based on what is given in the HELO string.

You don't need to do anything complicated.

As soon as your new ISP has told you what your new IP range will be you can request which PTR records should be put on each IP address.

Once you've moved, you can tell the old ISP that you no longer require them to maintain PTR records for you.

That's it. There's no "harm" in having both sets of IPs having the same PTR records on them for a while. So long as the IPs in use at the time have the right PTR records on them it doesn't matter that some other (now unused) IP address also has the same PTR record.