This is not an Xfinity Mobile issue. It sounds like you have an iPhone, so I suggest the following:
Go to Settings, Passwords & Accounts, Your Mail Account, Account, Re-enter your password, Done. Then go to your mail and see if it works.
If not working, do the same as above, but instead of changing your password, Delete Account, and then Add Account again from scratch.
If that doesn't work I would login to your email account on Safari or another device/computer and make sure it is actually up and running on the Internet.