Comment 78 for bug 603402

(In reply to Yves Goergen from comment #42)
> In case you're not aware of it anymore, going offline and back online does not resolve the issue. At least for me.
> The only thing that does help is restarting Thunderbird, which I have to doo every few days (snip)

Funny. It sounds that your Thunderbird is not standard one...
IIUC, IIRC, after Go Work Offline, Tb closes all connections, and after Go back Work Online, Tb executes login sooner or later, and after login, Tb opens Inbox(and others if requested) for new mail check by Biff, or open Inbox (or an Mbox) when selected, and upon first select Inbox/Mbox at a cached connection, Tb always issues "uid 1:* fetch Flags()" unless CONDSTORE is used. This is confirmed many times by getting IMAP log.
If "uid 1:* fetch Flags()" is used, Tb should know all flags of all mails in an Mbox.
If not, it usually means that your Thunderbird is not properly configured.

Have you checked IMAP protocol level flow by yourself?
Do you correctly disable CONDSTORE support as stated in this bug more than one time?
Or if Inbox, did you do required "click Inbox while other than Inbox, for example account, is selected at Folder Pane"?
Because "new mail check by Biff" is not immediately invoked after going back to Work Online"(invoked at next new mail check interval), "time lag" exists between "going back to Work Online" and "first new mail check after going back to Work Online".

> Thunderbird is the closest, unfortunately.
It's sad. We will have to see complants by you, which surely won't help problem resolving, many times in this bug....