How do I implement the reverse DNS lookup (PTR record)?

You need to contact the company which provided you with the dedicated IP address for your mail server on this subject.

PTR record indication in the direct zone enables you to independently manage the reverse conversion without the need to contact every time the provider who gave you the IP address. It is sufficient for you to write (in the reverse zone) the records pointing to the direct zone, and thereafter all changes will be made in the direct zone.

Let's assume that you need to write a record for mx1.test.ru host in the direct and reverse zone:

Records in the direct zone:

$ORIGIN test.ru.
test.ru. MX mx1.test.ru.
mx1.test.ru. A 192.0.2.1
1.in-addr.test.ru. PTR mx1

Record in the reverse zone (to be written once at DNS servers of the company which gave you the IP address):

$ORIGIN 2.0.192.in-addr.arpa.

The record is an alias of the record in the direct zone

1 CNAME 1.in-addr.test.ru.

Detailed information on using PTR records in the direct zone can be found here (item 5.2).