Resetting the (non-gmail) email application
If you don't mind having to completely re-configure it, and start as if you'd just installed that app:


Go to Settings → Apps
Scroll to the app in question, tap it
Select to "Delete Cache" and "Delete Data"


Now you've completely "refreshed" it. With some luck, the app somehow offers to export its configuration; if so, do that first, and re-import the settings after above described "reset" to save you from manually reconfiguring it.