Could you post the exact error message? Sending uses the SMTP server; POP (not Pop up) is for receiving. You might double check settings against https://www.xfinity.com/support/articles/email-client-programs-with-xfinity-email.
If the error message contains an email address, remove the userID portion but leave the rest. For example, if the message contained the email address "email@example.com", edit that so it reads "********@example.net" before posting.