I’d like to announce that sadly, we are going to be sunsetting IMAP support in Discourse. Neither we nor any of our customers use this, and the functionality has always been alpha-stage since release, with no further plans for future support.
We will be doing this within the next few months. If it proves easy enough, we will split out the functionality into an unsupported plugin for self-hosters, otherwise we will have no choice but to remove it completely from the codebase.
I will post further updates here when I start removing things.