You can try steps 2 - 3 - 4 from the following link: http://support.sprint.com/support/article/Troubleshoot-issues-related-to-email-on-your-HTC-One-M8/WTroubleshootingGuide_542_GKB67171-dvc8080007prd
Let me know if this helps.
Verify that the email address and password are correct.
On your HTC One M8 web browser, navigate to your email provider's website and try to sign in to your email account.
If you're unable to sign in, double-check that you entered the correct username and password.
If you recently updated your email account password, you also have to update the email settings on your HTC One M8.
Double-check that you have set up your email account correctly on your HTC One M8.
Be sure that you've entered the exact settings for each email account you want to access. The instructions are different depending on the type of email account you're trying to set up.
For a personal email address:
This is the most common scenario and includes all major public email providers such as Yahoo!®, Hotmail®, Gmail™, and AOL®.
If you've already set up the email address on your HTC One M8, try removing it from your device to delete any incorrect settings.
This step doesn't delete any of your emails. It only removes the link to your account for now. Remove an email account.
Add your email account to your HTC One M8.
For a business email address via Microsoft Exchange™:
Many companies provide a Microsoft Exchange email address to their employees.
Sprint recommends that you contact your IT department for the correct setup information. You'll need your server address, domain, username, and your corporate password.
After you have this information, set up the Microsoft Exchange account.
If your Exchange account still doesn't work, contact your corporate IT department for more help.
Perform a soft reset.
A soft reset reconnects your HTC One M8 to the Sprint network.
Press and hold the Power key and Volume Up key for 10 seconds.
Your HTC One M8 restarts.