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When some IMAP servers are upgraded and there is some downtime, BIS changes it's default behaviour for the affected email accounts and stops pushing emails in "real time". When an account is created, the BIS servers connect to the IMAP server and sit idle, allowing them to react instantly to changes (new email). When there is some downtime, BIS switches to a polling mode and a user doesn't get instant emails anymore, he gets emails every 5 to 15 minutes. Changing passwords or resending service books don't fix the problem. The only solution is to delete the accounts and recreate them. There has to be a better way, either automatic or that can be activated by a carrier support technician.
-- Olivier - interfaSys ltd Developing for BlackBerry 10 devices using the Sencha Touch framework.