Just to iterate what the others have said here, whether you have push email or not there is no need to keep opening the Mail app to check for mail, if you have set the auto-check times right, if you want near-push choose the lowest value, the icon will update automatically no matter what you're doing with your iPhone. It will send email as soon as you click send, which usually (I believe) triggers a "receive" check anyway, if not you'll not have that long before the iPhone undertakes its next scheduled send and receive check anyway.