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Setting the periodic synchronization interval to a value less than 5 minutes
Bring up the secret debug menu (see Gaia/Email/SecretDebugMode#Getting_to_the_Secret_Debug_Menu), then click the button labeled "Enable faster sync options". The account settings menus will now have a 20 second and a 60 second option for periodic mail synchronization until the next time you restart the app. (If you choose one of the options and restart the app, your choice will still be visible when you restart the app, but the other option won't be there and the setting will disappear if you change your setting back to one of the normal values.)