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Inbox Theory

DNS

Reverse DNS Lookup

Looks up the PTR record for an IP address — the hostname it reverse-resolves to, if one is published.

The PTR record for an IP — the hostname it reverse-resolves to, the opposite direction of a normal (forward) lookup.

Many mail servers refuse or flag mail from IPs with no PTR record, or where the PTR hostname doesn't roughly match the sending domain — it's a basic, widely-checked reputation signal.