Forward-Confirmed Reverse DNS (FCrDNS) is the foundational handshake requirement enforced by receiving MTAs worldwide. If your sending IP address does not resolve back to your SMTP HELO hostname, emails are immediately dropped.
1. The FCrDNS Verification Loop
- Receiving MTA Connects: Mail server receives an incoming TCP connection from sending IP (e.g.
192.168.11.32). - Reverse DNS Query (PTR): The receiver performs a PTR DNS lookup on the IP to discover the associated hostname (e.g.
mail.emailcampaigner.info). - Forward DNS Query (A): The receiver immediately queries the A record of
mail.emailcampaigner.infoto verify it points back to192.168.11.32. - HELO Match: Confirms the
EHLO mail.emailcampaigner.infogreeting matches the validated hostname.