2 points
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comes with the territory of encrypted email
AFAIK they haven’t tried to standardize their implementation, which to me implies that they’re not interested in interoperability. That’s unfortunate. I wouldn’t want to be locked in to a vendor like that.
At least some providers do try. FastMail published the spec for their modern, stateless replacement to IMAP through the IETF as “JMAP”, and built on top of existing RFCs where possible.