I don't believe there's anything in it that would prevent that. If you squint, the auth exchange looks a lot like SASL, and that supports pretty much anything.

I don't believe there's anything in it that would prevent that. If you squint, the auth exchange looks a lot like SASL, and that supports pretty much anything. This is something we'll gain more experience with at FastMail over the coming year. We'll be sure to share that experience.