Lots of threads on this topic. Everyone is waiting for the default app to handle this.
You can try "K-9" email beta, it supports IDLE (Push IMAP) I believe. I didn't like it as it doesn't have the "combined" Inbox which is more important to me than push.
Give it a whirl.
Not only is it periodic, sadly, it sometimes just fails to update for unknown reasons. Sigh.
Your only other real option is to forward email to GMail. Lots of people waiting on push, and I think it will come out eventually.