Hello, and welcome to AndroidForums. Have you tried to empty the application's cache in its Application Settings?
What you do is open settings, find and select Apps (may say Applications, or app settings depending on you device)
Once there you'll have a list of your apps, but you may need to go to the "all apps" tab in order to see your mail app. Find yo mail app in there.
Once you open the apps settings, you'll see a few options. clear the cache by tapping the clear cache button.
Keep in mind that if you wish to reply on this thread you will need to register, which is quick and easy unlike most sites. I would also suggest lurking in you handset's device-specific sub forum, here:
HTC EVO 4G - Android Forums
Or if yours is the LTE model, here:
HTC Evo 4G LTE - Android Forums
If you decide to register (you know you want to) then you will be able to reply here, or submit new questions in your device's sub forum. Hope to see your return!