# Choose the address to send a message to

> An inbox receives on a whole domain, which gives you one address per test without one inbox per test.
> https://facteur.eu/en/help/adresses-inbox

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](quotas-et-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.