What email client are you using? I've got the Xfinity Connect app on my iPad and have no issue when it asks me to log into my Xfinity/Comcast account. If you're using Outlook, you might need to edit your email account or delete/re-add it and enter your login credentials as requested.
No, this is not phishing, which is when an email is received requesting you to click on links or respond with private information. e.g. fishing for information to allow someone to get into your private accounts, etc.... SMTP is Simple Mail Transfer Protocol, an Internet standard for email transmission. When you set up an email client which uses SMTP, you have to provide the address of the server (smtp.comcast.net) and your email address/logon credentials.
How are you accessing your 'comcast.xfinity' account? Do you log in to the Xfinity/Comcast web site using a browser, open an email app (which one) or some other email client (which one)?