I was having the same problem (i use Comcast) - wasn't getting emails, said it was "checking" constantly. I came on here to find answers, meanwhile I simply restarted my phone and now it seems to be working fine. (I got all the test emails I'd sent from my computer all at once, and then it stopped "checking.") I literally just did this, so who knows if it will act up again, but try restarting and see if that helps.