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Read / Unread Status not Syncing with the Server

If your read/unread status is not syncing with the server, try these settings:

1) Preferences or Options / Accounts / [your account] / Advanced Button

* Uncheck "Use Idle Command"
* Set Max Server Connections To Cache = 1 (default is 5)

2) Then go to: Preferences or Options / Advanced / General / Config Editor

Paste the following into the Filter field: 
mail.imap.use_status_for_biff

And double-click it to change the status to False.

3) Restart Postbox.