The basic setting is “Settings->Mail->Accounts->Fetch new data”. Make sure “Push” is selected. Then each account will have an individual setting to pick: manual, fetch, or push. But, not every server supports push, so you might end up with a mix of things that will push, and others that fetch every 15 minutes.
The basic setting is “Settings->Mail->Accounts->Fetch new data”. Make sure “Push” is selected. Then each account will have an individual setting to pick: manual, fetch, or push. But, not every server supports push, so you might end up with a mix of things that will push, and others that fetch every 15 minutes.