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I think this may be an iOS 7 related issue, at least if you are using Gmail. I know some people have been having trouble accessing Gmail from iPhones for months, since Google apparently ceased support of activesynch/exchange at the end of January. You didn’t specifically mention that it was Gmail, so I’m making an assumption based on the number of people who have had an issue. You might want to try accessing your email through your browser and see if that works.
I did a little more looking around, and found this thread about iOS7 and push Gmail that you might find useful. http://forums.macrumors.com/showthread.php?t=1603501