> The SMTP has provisions that account for the host server being powered off But it's still built with the expectation that the recipient host will usually be

> The SMTP has provisions that account for the host server being powered off But it's still built with the expectation that the recipient host will usually be available, and that unavailability is a transient condition -- a failure to contact the recipient SMTP server is a noisy failure, and, after the first few retries, most servers back off to one attempt every 8 hours. Mail servers which are only sporadically available would require some fairly substantial rearchitecture.