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Use a temporary mailbox to inspect inbound email

Signup codes, magic links, and inbound hooks should not land in a personal inbox. A Mercur temporary mailbox is a receive-only @mercur-mail.com address in your workspace. You paste it into the app you are testing, then read the message in the console the same way you already inspect webhooks.

This page walks through create → receive → inspect, then optional JSON forward and MCP. Guest mode and the free plan cannot create mailboxes. You need Solo or Team — see Pricing.

1. Create a temporary mailbox

Open Mailboxes

Sign in, open Mailboxes, and choose New mailbox. You can give it a name (for example “Stripe test inbox”). Leave Forward to HTTP endpoint on Inspect only until you have a handler.

Mercur assigns the address. You cannot pick a custom local-part. It looks like this:

Example address
mb-aaaaaaaa-bbbbbb@mercur-mail.com

Copy the address from the console — the local-part shown there is yours and differs from the placeholder above.

Mailboxes are a workspace section, not a tunnel protocol. They do not count toward endpoint quota. Team members share the workspace addresses.

2. Send a message to it

Paste the address into the provider or app that must deliver mail: a signup form, a password-reset template, a “email us a receipt” field, or your own SMTP client.

Any client that can send internet mail can deliver to @mercur-mail.com. Unknown local-parts are rejected with SMTP 550, so only addresses you created accept mail.

A message you can send from a mail client looks like this (replace the To line with your address):

From: you@example.com
To: mb-aaaaaaaa-bbbbbb@mercur-mail.com
Subject: mailbox test
 
Hello from the temporary mailbox quickstart.

3. Inspect what arrived

Open the mailbox. New messages appear in the history. Select one to see:

  • headers (From, To, Subject, date)
  • parsed text and HTML
  • raw MIME
  • downloadable attachments

That is enough when you only need to confirm a code, a link, or a template.

Stored messages follow the same retention and per-mailbox item cap as request logs. See Limits and Security and data retention.

4. Optionally forward as JSON

If the app already handles webhooks, bind the mailbox to an existing HTTP endpoint in mailbox settings. After Mercur stores the mailbox copy, it POSTs JSON to that endpoint’s public URL:

  • type is email.received
  • mailbox id, address, and message id
  • from, to, cc, subject
  • parsed text and html
  • attachment metadata with contentStatus: "omitted"
  • rawAvailable when the MIME blob was saved

File bytes stay in the mailbox download, not in the JSON body. A failed forward does not drop the stored copy. Leave forward unset if you only need to inspect.

To receive that POST on your machine, forward the HTTP endpoint with the macOS app or the CLI agent.

5. Read mail from an MCP client

On Solo and Team, a connected MCP client can read the same mailbox without pasting JSON into chat. Typical flow: list_mailboxes → list_messages → get_message.

get_message returns parsed content and attachment names, not raw MIME or file bytes. Setup is in Remote MCP.

What your plan includes

Temporary mailboxes are paid. Guest and Free include none.

  • Solo — 1 mailbox, 10,000 emails per month, 8 MiB per message
  • Team — 3 mailboxes, 50,000 emails per month, 16 MiB per message

When the monthly email cap is reached, ingest refuses further mail (SMTP 452) instead of dropping it quietly. After a downgrade, extra mailboxes stay visible but stop accepting mail until you upgrade or delete them.

What a temporary mailbox is not

  • No send, reply, or outbound SMTP credentials
  • No guest inspector and no mailbox without an account
  • No custom local-part or catch-all
  • No public REST mail API
  • Not a production inbox for your customers

Delete a mailbox and that local-part is never reissued, including to the same workspace. The next mailbox gets a new @mercur-mail.com address. Retired and unknown addresses keep returning SMTP 550.

FAQ

Which plans include a temporary mailbox? Solo includes 1. Team includes 3. Guest and Free include none. See Pricing.

If JSON forward fails, do I lose the message? No. Mercur stores the mailbox copy first. Forward runs afterward.

Can I reuse an address after I delete the mailbox? No. Delete frees a quota slot; the next mailbox gets a new address.

Can an agent read mailbox mail over MCP? Yes, on Solo and Team, with the same OAuth grant or workspace API key as HTTP history. See Remote MCP.

Related

Create a temporary mailbox

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