SLAC

SLAC is Spam Limiter And Classifier: a Rust mail delivery agent for OpenBSD smtpd. It classifies mail with an OpenAI-compatible chat completions endpoint, adds X-SLAC-* headers, can quarantine spam to an mbox, and provides CLI/TUI tools for reviewing and correcting classifications.

SLAC is intentionally local and OpenBSD-focused. It is designed to be invoked by OpenSMTPD as an external MDA, read one message from stdin, classify it, then either delegate normal delivery to mail.local or append the message to a spam mbox.

Features

• OpenAI-compatible /v1/chat/completions classifier support.
• Conservative fail-open behavior: classifier/config/model failures deliver normally instead of losing mail.
• X-SLAC-* headers on processed mail.
• Optional quarantine to standard Unix mbox, defaulting to ~/spam.
• TUI and CLI tools for reviewing inbox/spam and moving misclassified mail.
• Correction history and message snapshots used as bounded prompt feedback.

Build

cargo build --release
doas install -o root -g bin -m 0555 target/release/slac /usr/local/bin/slac

Configuration

Start from:

cp examples/slac.toml /etc/slac.toml

Config search order is:

1. -c /path/to/slac.toml
2. ~/.config/slac/slac.toml
3. /etc/slac.toml
4. built-in defaults

The default example uses a local OpenAI-compatible endpoint:

[llm]
endpoint = "http://127.0.0.1:8080/v1/chat/completions"
model = "local-model"

Start with observe mode while testing:

[classification]
observe_only = true
add_headers = true

When observe_only = true, SLAC classifies and annotates mail, but always delivers normally. After you are comfortable with the classifications, enable quarantine:

[classification]
observe_only = false
spam_threshold = 0.95

[quarantine]
path = "{home}/spam"
require_verdict = "spam"

OpenSMTPD

Example smtpd.conf action:

action "local_slac" mda "/usr/local/bin/slac -c /etc/slac.toml mda" alias <aliases>
match for local action "local_slac"

See examples/smtpd.conf for a complete minimal example.

Validate and reload OpenSMTPD after editing:

doas smtpd -n
doas rcctl reload smtpd

Watch logs with:

tail -f /var/log/maillog

Modes

MDA mode is the default:

slac -c /etc/slac.toml mda < message.eml

Debug mode also logs to stderr:

slac -d -c /etc/slac.toml mda < message.eml

Test the configured classifier endpoint without delivering mail:

slac -c /etc/slac.toml test

The test command loads the same configuration as MDA mode, prints the endpoint and model settings to stderr, sends one synthetic classifier probe, and reports common endpoint mistakes such as using /v1 instead of /v1/chat/completions.

Review

The TUI provides split inbox/spam views, a message detail pane, and correction actions for moving misclassified mail. The screenshot below uses fictitious sample messages.

slac -c /etc/slac.toml tui
slac -c /etc/slac.toml list --mailbox spam
slac -c /etc/slac.toml show --mailbox spam --id 0
slac -c /etc/slac.toml move --mailbox spam --id 0 --to inbox --reason "false positive"

TUI keys:

• Tab: switch inbox/spam
• Up/Down or j/k: select message
• PgUp/PgDn: page through lists or preview
• Enter: open preview
• q: quit or close preview
• m: move selected message to the other mailbox
• r: refresh

Correction metadata is stored in ~/.local/share/slac/corrections.jsonl, with message snapshots under ~/.local/share/slac/corrections/messages/.

Moved messages keep their original receipt-time X-SLAC-* headers and receive separate correction headers such as X-SLAC-User-Correction and X-SLAC-Correction-Reason.

Safety Notes

• Do not configure a fake delivery command such as /bin/cat under real OpenSMTPD; OpenSMTPD will treat exit status 0 as successful delivery.
• SLAC does not bounce spam. Quarantine accepts the message and appends it to the spam mbox.
• Message ids shown by list and the TUI are scan indexes. Refresh after moves before reusing ids.