Apple Mail stuck on “Preparing to Move Messages”

In the last two weeks, Apple Mail is starting to get "stuck" with increasing frequency, showing a "Preparing to move messages: 1 of XXX" message in the lower left corner. Apple Mail still allows me to read messages, but attempts to delete or move messages fail as described below.

This typically occurs when I'm in the process of deleting messages from my inbox; Apple Mail cheerfully removes the message from the window, but rather than moving the messages upstream using IMAP, the transfer simply bumps the count of messages that are being "prepared." So, for instance, my Mail is currently stuck, showing "Preparing to move messages: 1 of 22". If I delete a message, that goes up to "1 of 23."

Turning network off and on appears to make no difference.

Quitting and restarting Mail "solves" the problem in the sense that it's no longer wedged, but all of the pending transfers are aborted, and my inbox suddenly again contains dozens or hundreds of messages that I already "deleted." Deleting then appears to work correctly… for an hour or two.

Relevant facts about my mail setup:
1) OS X 10.13.3
2) I'm using IMAP…
3) … to communicate with a machine running Dovecot.
4) "Connection Doctor" within Mail indicates no problems with the network connections, and I can refresh the status at will.

Many thanks in advance for any advice!