Nothing ships. No mail adapter, no inbox steps. There is an open issue proposing a Mailpit adapter and a set of inbox assertions; it has not been built.
The workaround I would push you towards is not "wait for the framework" — it is to stop routing the test through the mailbox at all where you can:
- if the verification token is readable from an API or from the database that issued it, read
it there and drive the link directly. You keep the assertion that matters (the link works, the account ends up verified) and lose only the assertion that mail was delivered, which is your provider's job more than yours.
- keep one scenario that does go through a real inbox, so delivery is covered somewhere, and
accept that it is the slow one.
If you genuinely have no such surface, run Mailpit in your test environment and write project steps against its HTTP API. That is exactly what the proposed adapter would do — you are writing it yourself a bit earlier, and it is maybe fifty lines.