JMAP was initially developed at Fastmail, but it changed shape quite a bit during the standardisation process at IETF, with significant input from multiple other providers. The end result was definitely the better for it. (In the end, RFC 8620 has two authors listed—one from Fastmail, one from Oracle.) JMAP’s good stuff.

(Source/bias: I worked on Fastmail’s webmail from 2017–2020.)