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What is Reverse DNS (PTR record)?
A DNS record that maps a sending IP address back to a hostname.
How it works in practice
Receivers look up the PTR of the connecting IP and, ideally, check that the hostname resolves forward to the same IP (forward-confirmed reverse DNS). Gmail requires it and rejects mail without it with error 5.7.25. The hostname should also match the one the server uses in its HELO greeting.
The common mistake
Leaving the default PTR from the hosting provider, which points at a generic name unrelated to your domain.
Check it yourself
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