Email App - Unable to validate account.
If you receive an error when trying to add an xfinity email account to your phone app that says, "Unable to validate account." then you have two factor authentication turned on. You will need to turn off two factor authentication temporarily to add your account to the phone app. To do this follow this guide located on the xfinity forum boards here: https://forums.xfinity.com/conversations/email/turn-off-two-factor-authentication/602daf9fc5375f08cd11f91e
Work Flow: Temporarily turn two factor authentication off, add your email account to the app, and then turn two factor authentication back on.
I fought with this for 15 minutes before I figured out what was going on. Hope it helps someone else who's wise enough to Google the issue.