iPhone 4S email not pushing

In the several years I've owned an iPhone, I have never had a problem with my Yahoo email pushing to my phone until recently. I have only had the 4s for a couple months and I'm not sure if it started immediately after upgrading from iPhone 3.

I have my email set to push every half hour, but it won't push unless I open the app. So I end up with a ton of unread emails at the end of the day, which is so frustrating. Another strange twist I noticed is that if I close the app from the recently used app bar (which I access by double tapping the home button and then close by holding down the icon until they begin to shake) my email pushes itself within seconds after doing so. I've tested it several times, so I know it's connected to this action. I've looked all over the Internet and saw thread about it being issues with Yahoo, but I'm not sure that's the issue here. Has anyone else experienced this, and if so, any solutions?