[bodyfour]

> the criticism is usually that IMAP does not require maintaining UIDs

Yes, exactly. UIDs should be sticky, but they should also be allocatable by widely distributed nodes without the need for central coordination.

If your UIDs are 32-bit monotonically-increasing integers then this is impossible. If they are 128-bit random numbers you get it for free. If you prefix them with a timestamp you even get reasonable ordering.

The entire UIDNOTSTICKY problem is a result of IMAP UIDs being so restricted.

I can see why a proposal as complicated as Lemonade failed to get traction, but I find the argument pretty thin. The "attractive nuisance" was the old open-relay SMTP infrastructure. If you were writing a POP client in 1991 you wouldn't think twice about sending mail since you just had to hit a SMTP server (didn't even have to be the "right" one!) and the mail would get delivered. When this all came crashing down far more effort got put into authenticated SMTP (talk about not separating concerns!) than would have been required to just get message submission right in the first place.