Try this little experiment: * Read an IMAP group (with nnimap, or any other client) * In a non-Gnus client, mark an article as unread * In Gnus's *Group* buffer, press 'g' * Note that the group with an unread article doesn't appear in Gnus as unread. Doing this the other way around works fine - if I mark an article unread using Gnus, it immediately appears as an unread article in the other client.