Try:
zimbraDataSourceMinPollingInterval is usually 60s or 1m or 2m & zimbraFeatureMailPollingIntervalPreferenceEnabled should be set to TRUE
To cancel polling, set the attribute value to an empty string ("") or 0.
Keep in mind that
zimbraDataSourcePollingInterval gets inherited from the COS to the account to the data source. If a child object sets the value to "" it inherits the parent's value - if set to 0 it's turned off.
Bug 22114 - Expose external account polling interval in UI (of course at an admin minimum time allowed to not increase the load too much...)