Solution 1:  Remove your gmail address from the mail app.  The gmail app (obviously) is specifically for gmail and, IMO, works much better.  I had this same issue when I first set up my Rezound.  It's kind of confusing the way they ask you for your gmail address twice.

Solution 2:  Have you tried clearing data from the mail app?  menu>settings>applications>manage applications, select the all tab at the top of the screen, select mail, press clear cache and clear data.  You will have to re-input any other email addresses into the mail app, but it's better than a factory reset.  If it doesn't work the first time, try disabling sync for the account.  menu>settings>accounts & sync, select the account, turn off sync.