PTR record and email anti-spam
A small portion of email systems use PTR record as an anti-spam mechanism. It does reverse DNS lookup to find the canonical name of the sending IP address to see if it matches the sender's email address. For example, if an email is sent from 12.34.56.78 and the sender's email address is john@foo.com, the email system will look up the PTR record of 12.34.56.78 to see if it is related to foo.com. If not, it will reject the email as spam.
There are several ways to minimize the chance that your messages are marked as spam
Add a TXT record like the following:
v=spf1 ip4:12.34.56.78 include:outlook.com include:_spf.google.com ~all
One can use Google's postmaster tool to monitor the spam situation of a domain.