Well gmail has 99.987% currently but only 99.9% is promised according to SLA.

https://support.google.com/googlecloud/answer/6056635?hl=en#...

And Protonmail offer 99.95% according to theirs (as of 2018, latest info I could find) — https://protonmail.com/blog/protonmail-reliability-sla/

99.95% is a high standard already: 365x0.0005x24 is 4.38 hours outage for a year. In our current world, 4.38 hours is certainly not enough time to, say, rebuild a DB from backup, if needed.

Yeah, especially when they're saying it's calculated on a calendar month, so you can't even "draw down" on previous months with 100% uptime if you have a serious outage.

Yes, but I assume the rationale here is that they will only have to refund that month and can reset the clock for the next month. So if they have serious downtime it won't cost them for the full year - just for that one month.

Yes, you fail over to a hot mirror, or if you really have to rebuild because something went nuts, do it from a recent snapshot.

Noone can gurantee any specific uptime. Any projection for the future, however qualified and propable, is still only a projection. GMail could go down tomorrow, for 24 hours or forever. (Not, of course, that such optimism is in way warranted).

Whoever downvoted the comment above start your own server, and we'll talk a decade later. The comment is correct; nobody can guarantee uptime. The can promise, there can be agreements, insurance, etc, but guaranteeing is basically impossible. Even if someone were to use a real mainframe cluster across the globe one still connects to them via international ISPs and those routes can go down as well.