Relay mode

Plain capture is great for tests. Sometimes you want capture and delivery in the same run — for example, while staging a transactional rollout where you want to read every message before it ships, but still want it to ship.

Relay mode does that.

#Quick start

1. Open Preferences with ⌘,.
2. Scroll to the Relay section.
3. Tick Forward each captured message upstream.
4. Paste the upstream URL into the Upstream URL field, e.g. smtp://relay.example.com:25.
5. Click Apply.

The capture port behaves exactly as before. After a message is captured, a background relay task picks it up and reships it to the upstream MTA. The upstream's response does not affect what the original sender sees: capture always returns 250. A relay failure is surfaced visually (the toolbar relay pill turns red) and the message stays in the inbox.

The toolbar Relay pill goes from Relay off to Relay relay.example.com:25 once enabled. Click the pill any time for a smaller standalone dialog that toggles relay without opening the full Preferences window.

#With authentication

Put the credentials in the URL:

smtp://alice:s3cret@relay.example.com:587

The userinfo becomes AUTH PLAIN credentials when the relay reaches the upstream. The credentials are stored as part of the settings JSON in your user-only Application Support directory.

#TLS

Use smtps:// for upstreams that require TLS:

smtps://relay.example.com:465

The connection is wrapped in TLS using the system trust store. If your upstream serves a dev / staging certificate that isn't in the trust store, tick Skip TLS certificate verification under Relay. That option is intended for local development; do not point it at production traffic.

#Hot-update

Relay settings are picked up without restarting the SMTP listener. You can flip the relay on, change the upstream URL, or turn it off mid-session — the captured-mail buffer is untouched, in-flight SMTP transactions complete, and the next captured message uses the new relay config.

#One-off Release

The Release tab on each captured message resends a single message to a specific upstream without touching the global relay setting. Useful when you want to capture mail in bulk and selectively forward only the ones you care about.

#Capture-then-forward semantics

The capture path always wins. If you want the original sender to see relay failures, use a real MTA — that's a different tool.