SMTP requires sending servers to retry for at least 4 days, so 98.9% uptime over a year is guaranteed to be sufficient so as to not lose any mail ... so that is

SMTP requires sending servers to retry for at least 4 days, so 98.9% uptime over a year is guaranteed to be sufficient so as to not lose any mail ... so that is "several" as in "one", I suppose? Of course you can have a lot more downtime than that, even just 20% uptime would be sufficient if timed right to ensure that no mail gets lost. Or in other words: My central heating system has higher availability requirements than my mailserver, so I guess everyone in non-tropical places has a non-trivial endeavour going?