Ondine wrote:

I think I didn't describe what happens very clearly. Suppose I have
20 unread messages in an Inbox with around 200 messages in total.
I'll go to the 'Unread' folder, which will display the 20 unread
ones. I'll select one, right-click and choose 'Mark as read'. Then,
almost ALL of the unread messages will disappear from the folder, and
I am normally left with just the message which I selected and maybe
one or two newer ones - so the message which has been marked as read
actually stays in the Unread folder and all the others, which I hadn't
touched, go. Then, when I return to my inbox with its 200 or so
messages I'll find that a whole load are no longer there. I've not
checked but it might be that the messages which disappear are all
those newer than the one I selected to be marked as Read - but I can't
confirm that.

I can't reproduce this, but I'm not a big user of IMAP. I do have a couple
of IMAP accounts, but I get messages so rarely delivered to them and have so
few messages in them that I don't think I can help. Sorry.
--
Brian Tillman [MVP-Outlook]