The message status lifecycle is the forward-only sequence an SMS moves through at a gateway: queued, sent, then delivered, failed or expired, or rejected when refused before sending.
A message is queued when accepted and priced, sent when handed to a carrier (the billing point), and reaches one final state: delivered on a handset receipt, failed when the carrier could not deliver, expired when validity ran out. Rejected is the sixth state, for messages refused before any attempt, which are never billed.
Statuses move forward only. A replayed earlier event must not overwrite a later one, which is why webhook handlers order by time and precedence.
At SMSMeteor: the six statuses are queued, sent, delivered, failed, expired and rejected; test keys return a seventh, test, for messages that were validated but never sent.