My workstation runs 24*7 and one of the servers it uses for IMAP has a
certificate name that doesn't match.  I tell it to "trust forever" and it
works.  Then the server gets a new certificate (part of an automated
process) and it gives an error (which is good).  But when this happens
while I'm not at my desk it keeps polling the IMAP server and giving a new
message dialogue every time it fails the check, which leads to CTRL-ALT-ESC
being required as the only way to close all those windows.  I'll send a
screen-shot after the bug number is assigned.

I think this should only give one error dialogue about it and should
refrain from talking to the IMAP server after that until the user
responds to the dialogue.

Here is a screenshot.  Also plasmashell crashed at about the same time, I
don't know whether hundreds of dialogue windows about this error contributed
to plasmashell crashing.