Used posteo for 4 years. Pretty good, rules engine is better than a lot of other ones. Support also answers fast.
A few time-out or slow moments probably because they have too many clients. Visible only during a minor part of my 4 years.
But some people didn’t receive my mail, which made me change mail service.
I don’t see how it replaces anything without audio & video calls. Doesn’t mattermost already cover this scope…? And a lot of other open source softwares… For me, the rarest and dearest feature is these fixed call channels you have in discord. Matrix and element have them, but their ui is shit. I’m waiting for it to be implemented on Cinny, there is a PR for that.