Initially, the proposition relied on Fastmail protocol, which was/is superior to IMAP. It was pleasing that standardization began, as MS Activesync charged a few dollars per user just for the protocol. Even the Apache James mail server supported it as a draft. Despite becoming a standard make it better , but lost momentum.

Still hope at least both will add push notification to 3th party apps. And a BIG hope for https://stalw.art/

+1 for Stalwart