I'm having problem with one of my target email recipient, when I tried to send email to firstname.lastname@example.org, I got this error message from my email server:
Diagnostic information for administrators: Generating server: mail.domain.com email@example.com mail.abacusevents.com #<mail.abacusevents.com #5.0.0 smtp; 554 "Refused. You have no reverse DNS entry. Contact firstname.lastname@example.org for details."> #SMTP#
which PTR records should I add into my intenral DNS or my ISP DNS server ?