The shortest interval to download user email with the Exchange Pop3 connector is 15 minutes. Too long for most people. To shorten that to 5 minutes, you have to update/add a registry value in this key:
It works by dividing the default schedule interval by that value. For example, the server connector is set to 15 minutes, the lowest setting. Set the reg value to 3, so 15 divided by 3 equals 5. This makes it a 5 minute interval.