Why does my Gmail not notify me when email comes?

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http://forums.androidcentral.com – I checked my gmail sync and also google account sync and check the sync on gmail. But why when emails are coming, emails are not pushed to the galaxy tab immediately? I have to sync manually in order to know new emails. My PC desktop outlook notifies me immediately. I think the gmail app will notify users as email arrives at the very same moment, right? (HowTos)