Which mails one account has been queued, newest first. There is no mail capture in this repo, so
a system test asserting a message was composed reads the queue instead of an inbox.
Queues a test send of an unsaved template body to the account’s own address, unless the
account already queued max_per_minute in the last minute, in which case returns None.
placeholders must hold every substitution: the sender derives none for a test send.
True once a delivery attempt has failed for good: not retryable at all, or retryable but its
retry window has expired. Shared with credit_registration_account_linking_emails.rs so the
two cannot drift on what counts as a hard failure.