You need to enable the account in a desktop email client like Outlook, Mac Mail, or Thunderbird, etc. Then use a backup utility to back up the client on the external drive. I use Mac Mail since I'm on a Mac.
When I "enable the account" in Outlook, does that mean that I will have two email systems to manage? I have often wished I had Outlook instead of Comcast, but never dared (or knew how to) switch for fear of losing saved emails and contacts.
An email client like Outlook is a program you download to your computer, or it may have been there already depending on how you bought your computer. It's part of the MS Office suite of productivity programs. Comcast is the email provider. All the client does is let you manage the account without having to use the webmail interface if you don't want to. There are a lot of advantages to using a desktop client. And no, you don't lose anything in the account by using one. They are especially nice when you have multiple email accounts to manage. I have 11 email accounts right now from Comcast, iCloud, Gmail, Yahoo, Outlook.com and Mail.com. I have them all configured in Mac Mail.
dana_bryan, latoque is right--using a desktop client allows you to manage your Comcast emails the way you wish-- especially if you're using IMAP instead of POP3. Everything would flow from the outlook box as if you were using the Comcast email platform via XFINITY connect.