In-flight login mail is dropped on shutdown #159

RequestLoginLink hands mail.Send to a goroutine so that a hit and a miss return on the same path — a synchronous SMTP round trip was a timing oracle that answered whether an account exists (#155).

Nothing drains those goroutines at shutdown, so a restart during delivery drops the mail silently. The cost is one of the account's five hourly slots and a retry, which is why it was not fixed in !261.

Ref #155.