This happens to me from time to time. It will do that for a week or two and then stop; then some time later it will start doing it again. In fact, my phone hadn't done that for several months until a few days ago and it has been doing it now for those past 3 days. It is annoying. The thing is that I haven't changed any of the settings on my phone. Although the suggestion to delete the email account and then set it up again sounds reasonable I haven't done that in the past when I had this problem and it corrected itself. I guess I'll just wait until it stops doing it again.
As for the suggestion to set up the email account to delete mail from the server, although that would work, I want all my email to go to my desktop which is where I keep my emails. If I set up my phone to delete emails from the server and I receive emails on my phone when out there's a good chance those emails will never show up on my desktop computer at home. That's because my phone and desktop check for emails at different times/intervals so if my phone receives an email before my desktop it will be deleted from the server before the desktop has a chance to retrieve it. So, I'd only suggest that solution if you don't care if the emails go to another computer.