your mailserver needs a PTR record
Some mailserver admins think that it is better to leave out the PTR record or just forget about it. The PTR record is necessary because it tells the other mailservers that your mailservers IP is responsable for the domain in the emailaddress. (reverse DNS). If you don't have one, other mailservers will think - sometimes automatically that the mail is coming from an infected pc.
You will need that PTR record if you want to install a SPF record that would authenticate your mailserver and mail.
And if you think that you don't need this stuff, than tell your users why their emails are being refused by other mailservers for no reason and hope that they don't do any research themselves.