Use cases

A handful of recipes for things you'll do over and over.

#Stop polluting test inboxes

Most apps need real-looking SMTP credentials in dev to function. Pointing them at MailBox Ultra means every outgoing email lands in the local app instead of dev@yourcompany.com (and worse, occasionally a real customer when someone forgets to suppress sends in staging).

MAIL_HOST=127.0.0.1
MAIL_PORT=1025
MAIL_FROM_ADDRESS=app@local.test
MAIL_ENCRYPTION=null

#Transactional mail QA

Render a password-reset email, an invoice, an order-shipped notification. Compare HTML and text alternatives, scrutinise headers (Reply-To, Auto-Submitted, List-Unsubscribe-Post), download attachments from the Attachments tab, view the raw RFC 822 source for issues a parser would smooth over.

The HTML tab paints captured email through WKWebView, the same engine Mail.app uses, so what you see is faithful to how the real client renders it.

#Responsive HTML preview

The HTML tab has three width buttons:

• Mobile (390) — iOS Safari User-Agent, 390 px viewport. @media (max-width: 600px) rules fire.
• iPad (834) — iPadOS Safari User-Agent, 834 px viewport.
• Desktop (1280) — desktop Safari User-Agent, 1280 px viewport.

Switching widths swaps both the viewport and the User-Agent so server-side responsive frameworks (Mailchimp, MJML, hand-rolled @media) trip the same paths the real client would.

#Reverse-engineer a sender library

Curious what your queue worker actually emits? Point it at MailBox Ultra and inspect:

• Does the From header match the envelope sender?
• Is the message multipart/alternative or just one body?
• Are attachments attached or referenced (Content-Disposition)?
• What Content-Transfer-Encoding does it pick?

The Source tab is the source of truth. Everything else is parsed for your convenience.

#Capture-and-deliver staging

Want to read every email a staging environment sends and still send them out for real? Open Preferences (⌘,), tick Forward each captured message upstream under Relay, paste an smtp:// or smtps:// URL, click Apply. Capture happens first, relay happens after — if the upstream is down, the message stays in the inbox and you can resend it manually from the Release tab. See the Relay page for the full story.

#AI-assistant pairing

Hand a coding agent a tail-able feed of mail your app is sending while you keep working in the GUI:

1. Open Preferences with ⌘,.
2. Tick Append every captured message as NDJSON to a log file under Logging.
3. Pick a path, e.g. /tmp/mail.ndjson.
4. Click Apply.

Then in the assistant's tool config, give it tail -f /tmp/mail.ndjson | jq so it can react to subject lines, attachment counts, or specific recipients in real time. Add Forward each captured message upstream under Relay if you also want the messages delivered for real.

The Logging page documents the JSON shape on each line.

#Replay a flow

Capture a signup email once. Open the Release tab on that message, point at your dev SMTP, and the same captured message goes out again. Useful for clicking confirmation links a hundred times without burning through verification quotas.

#Onboarding test runs

Run the app, send mail through it from your test suite, then either inspect by hand or point a script at the NDJSON log file. The Logging page shows the JSON shape — id, received_at, envelope_to, subject, headers, etc., one object per line. The shape is stable across patch releases and any breaking change is called out in the changelog.

For a totally clean inbox between runs, hit ⇧⌘X (Clear inbox) or just relaunch the app.

#Devcontainer / docker host

If your dev container runs on the same Mac, set the bind address to 0.0.0.0:

1. ⌘, to open Preferences.
2. Set Bind address to 0.0.0.0.
3. Apply.

The container then reaches the SMTP listener at host.docker.internal:1025 (or whatever host.docker.internal resolves to on your host).