Markle wrote:

I checked user1's account settings and saw that there's a dormant pop
account configured from back when there was another unrelated problem
with the system. I got pretty excited at this point, I thought you
may well be on to something. But then when I checked user2's account
settings only the IMAP account affected is configured and listed.

Off-hand, I can't think of what may be happening, then. If I do think of
something, I'll post it.
--
Brian Tillman [MVP-Outlook]