Posthorn – A self-contained email pen-pal daemon for slow, self-hosted LLMs

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posthorn#

A self-contained email pen-pal daemon for slow, self-hosted LLMs.

You send an email to model@provider.domain over SMTP. posthorn queues an LLM completion, calls an OpenAI-compatible gateway, and delivers the reply to your maildir — readable via IMAP from any standard mail client (mutt, Thunderbird, etc.). Conversations thread correctly across round-trips.

How it works#

┌────────┐ SMTP ┌─────────┐ enqueue ┌─────────┐ HTTP ┌──────────┐ │ client │ ──────► │ smtp.rs │ ────────► │ llm_jobs│ ───────► │ gateway │ └────────┘ └─────────┘ └─────────┘ │ (ollama) │ ▲ └────┬─────┘ │ IMAP │ chat │ ┌─────▼─────┐ ┌────┴─────┐ read ┌─────────┐ write ┌─────────┐ poll │ worker.rs │ │ client │ ◄────── │ imap.rs │ ◄─────── │ maildir │ ◄───── │ + lettre │ └──────────┘ └─────────┘ └─────────┘ └───────────┘

SMTP (src/smtp.rs) — minimal receiver; no AUTH, no TLS. Accepts mail to model-id@provider-id. (with + standing in for : in the model-id, so model+tag reaches model:tag), parses headers/body, resolves the thread via In-Reply-To/References, and stores the message in maildir++.

Worker (src/worker.rs) - runs as a short-lived child process , one per job. The serve dispatcher claims pending llm_jobs and spawns posthorn worker --job-id N; the child builds the chat history from the thread, resolves the provider's gateway by name, calls it, and composes the reply email (correct Message-ID/In-Reply-To/References so clients thread it).

IMAP (src/imap/) — IMAP4rev1 server using imap-codec for spec-correct parsing and encoding. Supports SASL AUTHENTICATE PLAIN, SELECT/EXAMINE, FETCH (with BODY.PEEK[HEADER.FIELDS ...] subsetting), STORE (\Seen/ \Deleted), EXPUNGE, CLOSE, SEARCH, IDLE, and LIST.

Storage — maildir++ on disk for raw .eml files.

IDLE push (live new-mail notifications)#

Instead of polling, IDLE uses inotify to watch the user's maildir and pushes * N EXISTS the instant a message lands — so mutt shows new replies without a manual refresh. No polling, kernel-driven.

Requirements#

Linux (inotify is Linux-only)

Rust (edition 2021, MSRV 1.84)

PostgreSQL 15+

An OpenAI-compatible gateway (e.g. Ollama)

Build#

cargo build -r Configure#

posthorn reads a TOML config (figment; also overridable via POSTHORN_* env vars). Example (test/config.toml.example):

[daemon] domain = "posthorn.localhost" data_dir = "test/data"

[imap] host = "127.0.0.1" port = 11143

[smtp] host = "127.0.0.1" port = 11025 max_message_bytes = 10485760

[llm] default_system_prompt = "You are a helpful test assistant. Keep replies short." default_provider = "ollama"

# One block per OpenAI-compatible gateway. The `name` is the subdomain used # for routing: mail to `model-id@.` is sent to that gateway. [[llm.providers]] name = "ollama" gateway_url = "http://127.0.0.1:11434"

[[llm.providers]] name = "llama-cpp" gateway_url = "http://127.0.0.1:8080" # api_key_env = "LLAMACPP_API_KEY" # optional; read from env at request time

[[users]] email = "user@example.com" password = "hunter2" Users declared under [[users]] are...

Posthorn – A self-contained email pen-pal daemon for slow, self-hosted LLMs

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