I've spent the last month implementing and testing IMAP support for my app [1].
It has... not been very fun.
Before we were limited to providers with a REST API, so implementing IMAP should in theory allow us to support a much wider range of email servers with a much lower support burden.
Although the protocol spec is tight, it seems like some providers randomly bug out or hang [2], respond with custom error messages, offer bespoke functionality [3], or generally don't comply with the spec in breaking ways [4].
As a result I'm more worried about this going forward than I had hoped.
I'll now also happily support anything that can help modernize email, so I'll apply to the developer program and will be watching JMAP closely.
[1]: https://leavemealone.app/
[2]: Yahoo Mail always down (https://downdetector.com/status/yahoo-mail)
[3]: https://developers.google.com/gmail/imap/imap-extensions