http://en.wikipedia.org/wiki/Push_email
Android’s built-in Gmail client uses Google Sync to push email for Gmail accounts set up to sync with the phone. Android also supports Microsoft Exchange accounts natively through its default mail application. Emails arriving into the Microsoft Exchange inbox are instantly pushed to the device. Calendar events sync both ways between exchange and the device.
Yahoo Mail is not pushed to an Android device as android is still lacking in IMAP IDLE, see below. An alternate to lack of native support for Yahoo Mail is to install the free Yahoo Mail app which provides instant push email.
Recently also Hotmail has been made push configurable for Android smartphones through the default mail application.
K-9 Mail, a third-party application for Android, provides IMAP IDLE support.