Stop updating dns record
Most ISPs have a DNS Services department which can create a PTR record for you.
Knowing the A record for mail.example.com, you must create a reverse PTR record.
For example, you might log into the web portal for the domain and create an "A" record for "mail" for 192.0.2.21.
This would create a mail.published on the Internet.
Some common DNS Hosting providers consist of companies such as Go Daddy, Network Solutions and Dyn DNS to name three.
There are possibly hundreds, so shop around if you have not found one yet.
For the most part, the web interface will be similar, however, at a dynamic DNS hosting provider. You will first need an A record for your mailserver.