Crate mailrs_shield 

SMTP anti-spam primitives: DNSBL, greylisting, and FCrDNS.

Three independent modules used by inbound SMTP pipelines to filter unwanted mail at connect / sender-evaluation time:

• dnsbl — query DNS-based blocklists (Spamhaus, Barracuda, …) for an inbound client IP, with an in-process TTL cache.
• greylist — temporary 4xx-defer policy (Harris 2003 / RFC 6647): unknown sender triplets get one initial defer; legitimate senders retry, spammers don’t. Comes with an optional Redis-backed store (greylist::GreylistDb) behind the default redis-store feature.
• ptr — forward-confirmed reverse DNS (FCrDNS) score for an inbound client: 0.0 if PTR → forward roundtrip matches the EHLO domain, 1.0 if it doesn’t.

Feature flags

• redis-store (default) — enables the Redis-backed greylist::GreylistDb store (with optional Postgres cold backup). Disable to plug in your own store.

Modules

dnsbl

DNSBL reverse-IP scoring helpers (Spamhaus + generic lookup interpretation). DNSBL reverse-IP scoring re-exported from mailrs-dnsbl.

greylist

RFC 5321-style greylisting: defer-on-first-seen with optional Postgres + Valkey storage. Greylisting policy + an optional Redis-backed store.

ptr

Forward-confirmed reverse DNS scoring (FCrDNS) for sender-IP reputation.