backing up email to computer

The simple way?  Add your email account to the Mac mail program, but don't use the IMAP server, use the POP server.  POP mail makes a local copy.    If you access your mail from a second device and delete things using the IMAP server, those changes will sync when you check  your mail on the 1st device.  It won't with POP.   If Xfinity's server eats your mail or loses it, you'll still have a local copy that won't get wiped out when you Sync mail.

You can actually have both setup both at the same time if you want.  The server settings are here:

https://www.xfinity.com/support/articles/email-client-programs-with-xfinity-email 

You have to enable 3rd party mail client access from web mail first:

https://www.xfinity.com/support/articles/third-party-email-access 

Then, make a backup with Mac's TimeMachine.  Do it regularly and you won't lose your mail.

... The simple way?  Add your email account to the Mac mail program, but ... use the POP server ...

Agree, but also be sure to turn off "delete messages from server" (or enable "leave messages on server") in the Mail program. Also, on https://connect.xfinity.com/, set "(gear icon) / Email Settings / Mail / Advanced Settings / POP Settings for Other Apps" to "Keep" to prevent POP from removing messages from the Inbox when you fetch them.

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I’m not completely computer savvy but think I follow you here! Thing is I want to download an entire batch of emails from the past 5 years that I can save for example to an external back up drive. Is that possible?