Naturallymessagesyou read will "disappear" from the Unread Mail folder.
However, they shouldn't vanish from the Inbox unless they're marked for
deletion. *Are you subscribed to theIMAPfolders? *Do you download the
entire message and not just the headers?

Franky, I find Outlook'sIMAPtransport to be less than robust.
--
Brian Tillman [MVP-Outlook]

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.

Fortunately the messages aren't actually being deleted on the imap
server so they do return, but then I get the temporary duplicates.
I'm sure it's a synchronisation issue.

I am subscribed to the IMAP folders and generally don't have problems
unless I meddle with the Unread folder. And yes, I do download the
entire message.

Thanks for your help so far.

Ondine.