I’m not sure what the reasoning was with not including it as it is an HTC app, but the fine folks over at XDA Developers have made it easy to add it on.

Supposedly this is a carrier decision and not one made by HTC. Any other company names, product names, service names and logos referenced in connection with the Application may be the trademarks of their respective owners.Here’s an app that probably should have been included with your HTC One M8, but wasn’t. HTC, the HTC logo and other HTC product and service names referenced in the Application are the trademarks or registered trademarks of HTC Corporation in the United States and other countries. HTC Mail uses BIND_DEVICE_ADMIN permission in order to grant device administrator privileges that are requested by certain email services, like Microsoft Exchange. Support for Exchange ActiveSync policies Create meeting invitations without leaving HTC Mail Text Reflow auto adjusts content to fit your screen at different levels of zoom Filter search results by sender, receiver, priority or attachment Search for messages by subject, sender, receiver or content and all fields Change your message text’s font, color, highlight and more Switch between your accounts and folders from the drawer Quickly add accounts with the mailbox setup wizard Keep your inbox in check with the easy, intuitive HTC Mail. It’s simple to add accounts, compose messages, switch between folders and accounts or search for a message. HTC Mail organizes all of your email accounts in one place thanks to compatibility with Gmail, Yahoo Mail, Outlook, Exchange ActiveSync and other IMAP or POP3 accounts. An easy, intuitive way to organize your email accounts.

Download htc email app