Choose the address to send a message to

An inbox does not receive on one address: it receives on a whole domain.

anything@<inbox-identifier>.inbox.facteur.eu

Everything before the at sign is yours. Send to qa@, customer-42@, signup+3f9a@ or signup-2026-08-07@: all four messages land in the same inbox, and each one keeps the exact address it was sent to, in its to field.

Give every test its own address

Two concurrent runs sign two accounts up, and both confirmation emails land in the same place. You then have to guess which one is yours: by subject, by timestamp, or by a sleep.

Build one local part per run instead:

signup+<run-id>@ab12cd34.inbox.facteur.eu

Each test then reads its own message by filtering on to. You have no tests to order, no arbitrary wait to sit through, and no inbox to create and tear down every time.

It is also why your plan’s inbox ceiling rarely gets in the way: one inbox per test suite is almost always enough. Inboxes are counted per organization rather than per workspace, see Quotas and retention.

What the receiving server accepts, and what it refuses

What you send	Its answer	What you do
To a known inbox, quota available	Accepted, stored, parsed	Nothing, read the message
To an inbox that does not exist	SMTP 550	Check the identifier: no inbox is ever created on the fly
While the monthly quota is exceeded	SMTP 5.7.x	Buy a pack or wait for the reset. The message is neither stored nor parsed
To a recipient outside the inbox domain	Refused	Fix the address: we relay no mail

A server that accepts mail for a recipient it does not know ends up on blocklists, and every customer’s reception goes down with it. That is why the refusal is immediate rather than silent.

The inbox domain is not the site’s domain

Inboxes live under inbox.facteur.eu, never under facteur.eu. A domain that receives test mail builds a sender reputation of its own, and mixing it with the domain that writes to you would make our email depend on your tests.