LINUX | Reverse DNS and Junk Email

Many email servers depend on Reverse DNS in order to identify if the mail is legit or spam. For the Reverse DNS to work correctly, two points need to be addressed
a) Look for IP address of the mail server should return its name. PTR record should be set for this
b) Reverse look on the name should return back the IP address

For example,

$ dig -x 217.83.122.34

;; ANSWER SECTION:

34.122.83.217.in-addr.arpa. 66372 IN PTR mail.example.com.

$ dig mail.example.com

;; ANSWER SECTION:

mail.example.com. 900 IN A 217.83.122.34

Check the IP address that is getting resolved matches the name that is getting returned back.