It was quietly announced in June of 25: Comcast -- aka Xfinity -- is moving all its' customers email over to Yahoo's servers. They are doing this in shifts, and at some unspecified date in the future they plan to exit the mail server business altogether.
This will of course affect all customers (and former customers who still use an @comcast.net email address). There is some wiggle room: Not all customers will be moved at once, and customers cannot change over to the new servers until they receive an invitation. Once received, the invitation need not be responded to immediately; at this writing Comcast has not declared a final deadline but indicated that, if the customer never does migrate, eventually their email will stop working. Once migrated, the customer will only be able to use Yahoo's platform, cannot migrate back, and Comcast Xfinity support will not be able to assist them with things such as password resets. Customers WILL keep their email address(es), contacts, and old mail through the transition when it is handled correctly.
The details are linked below from both Comcast's and Yahoo's side, with FAQs and support resources listed out; savvy customers may well be able to handle this on their own. Creative Goose is prepared to assist with this transition. We're here to back you up, or we can make a remote support appointment to do this with you, every step of the way.
Comcast Xfinity page on the Email Transition, with FAQs
Yahoo Welcome page for Xfinity customers migrating over, with support info.