We have become aware of a problem that seems to be affecting a small number of customers. The symptoms are that new mail in the Inbox does not appear to synchronize with email programs via IMAP, while other folders continue to synchronize as normal. However, the messages that do not synchronize can be accessed without any problems using the webmail interface.
UPDATE: There is now a fix available for this problem; please contact support at https://support.runbox.com if you are still having issues.