If you are on a Mac, the easiest way to back up your email would be to enable the account in Mac Mail. It's the desktop email client that comes as part of Mac OS. It's also integrated with all things Mac, including the built-in Time Machine back-up utility. So if you let Mail configure the account for you it will be with the IMAP protocol. (Comcast email is IMAP compliant by default) That being the case, I would create local folders in the section of Mail labeled "On my Mac". Then move the messages you want to save into those local folders. Once that is done you could then start deleting them from the webmail interface to reclaim space on the Comcast servers.
And when using Time Machine you will want to use an external drive for the back-ups. Time Machine will automatically create back-ups on a schedule, and recovering emails is pretty easy should the need arise.
Good morning Latoque,
Thank you for taking the time to answer my question, I appreciate this. Your explanation is very helpful and will follow your advice.
Thank you and have a great day.