This creates a dual-stack operational environment. They always have
real -- and often problematic -- operational effects.
Well, we've survived with POP and IMAP for two decades.
We might want to put something in the charter noting that the
underlying mail has to be RFC822/5322, but the drafts all seem to
assume that.
Re separating mail pickup from submission, I understand why we decided
to keep it separate when we did RFC4409, but that was a decade ago and
the mail world has changed. In particular, I don't think anyone in
2006 expected that the majority of mail users would be sending and
receiving mail using apps on the phones in their pockets.
At this point, if someone says that their POP/IMAP/JMAP and SUBMIT
servers need different credentials, I'd suggest that is a cruel joke
to play on your users, you should fix it, and while you're at it,
you should accept submission through JMAP, too. If you can't fix
it, there will still be some IMAP and SUBMIT clients for a long time.
R's,
John