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MTA Hooks

An HTTP-Based Mail Processing Protocol

draft-degennaro-mta-hooks-00: IETF 125 DISPATCH Mauro De Gennaro: Stalwart Labs LLC

Why This Work Matters

Mail filtering is critical infrastructure: spam filtering, virus scanning, policy enforcement: yet there is no standard protocol for MTA-to-filter communication.

  • Milter (Sendmail, ~2000) is the de facto solution but:

    • No formal specification: only a C library (libmilter)
    • Proprietary binary wire format, painful to implement in modern languages
    • Inconsistent behavior across MTAs (Sendmail vs. Postfix)
    • No support for outbound delivery hooks
    • Not adopted by modern MTAs

  • Filters like Rspamd, ClamAV, SpamAssassin all rely on Milter over the network today: on an undocumented protocol

MTA Hooks proposes a standardized, HTTP-based replacement that is simple to implement, language-agnostic, and covers both inbound and outbound mail processing.

Protocol Overview

MTA Hooks uses HTTP POST requests from the MTA to registered scanner endpoints at each SMTP processing stage. Scanners respond with JSON Pointer patches to modify the message transaction.

Key Design Choices

  • Built on HTTP: works with existing infrastructure (load balancers, proxies, monitoring)
  • JSON and CBOR serialization
  • Message representation based on JMAP (RFC 8620 / RFC 8621)
  • Modifications via JSON Pointer (RFC 6901) set/add/delete ops
  • Capability negotiation via discovery & registration

Inbound Stages (reception) connect → ehlo → mail → rcpt → data

Outbound Stages (delivery) delivery → defer → dsn

Actions: accept, reject, discard, quarantine, disconnect (inbound) / continue, cancel (outbound)

How It Works: Simple Model

MTA                                 Scanner
 │  POST /hooks/scan                │
 │  { "stage": "data",              │
 │    "action": "accept",           │
 │    "envelope": {...},            │
 │    "message": {...},             │
 │    "senderAuth": {...} }         │
 │ ──────────────────────────────►  │
 │                                  │  ← Analyze message
 │  { "set": [                      │
 │      {"path":"/action",          │
 │       "value":"reject"},         │
 │      {"path":"/response",        │
 │       "value":{"code":550,...}}  │
 │    ] }                           │
 │ ◄──────────────────────────────  │
  • Scanner receives full context (envelope, parsed message, auth results, TLS info)
  • Responds with patches: no need to echo back the entire message
  • Multiple scanners chain sequentially; terminal actions stop the chain
  • Both structured (message) and raw (rawMessage) representations supported

Prior Work & Community Interest
Draft draft-degennaro-mta-hooks-00
Presented IETF 123 (Madrid): MAILMAINT session
Discussed FOSDEM 2026
Implementation Stalwart Mail Server: in production
3rd-party scanners Multiple scanner implementations on GitHub
Rspamd Plans to implement MTA Hooks support
Interest 6–10 people expressed interest at IETF 123 and FOSDEM in participating in a WG

Feedback from IETF 123 and FOSDEM has already shaped the current draft: including support for both raw and parsed message formats, CBOR serialization, and the JSON Pointer modification model.

Hoped Dispatching Outcome

We believe there is sufficient interest and a clear gap to fill.

We ask DISPATCH to consider:

➊ Propose a new focused Working Group

or

➋ Hold a full BOF at a future IETF meeting

to advance standardization of an HTTP-based MTA filtering protocol.

Questions? Feedback? Mauro De Gennaro: mauro@stalw.art: https://stalw.art