Is email the only issue? Do text messages work ok? SMS?
1. Check your settings in Mail,Contacts,Calendar to be sure the server info is correct.
2. Delete your email account, try a hard reset (reboot). Hold the top power button and the home button at the same time until the Apple Logo appears, then release. Add your email account back in. Not work? Then...
3. Try Reset Network Settings. Go to Settings>General>Reset> Tap Reset Network Settings note: You may need to enter your wifi passcode back in afterwards.
If none of this works, contact your provider and explain to them your problem. Good luck!
Sent from from my ancient but trustworthy iPhone 5