It sounds like one, but we can't be sure without having more info about the message.
If you check your email with Comcast's Xfinity Connect webmail (https://connect.xfinity.com/), look for the XFINITY Verified Email logo next to any message that claims to be from Comcast/Xfinity. See https://www.xfinity.com/support/articles/comcast-verified-email. If the logo is missing when you view the message in webmail, the message is not from Comcast.
Unfortunately, AFAIK the logo is not present when you view one of their messages in an email app or program, not even in Comcast's own email app. It's only visible when you use their web email site.