I have thousands of them.
How do you access your email account, web mail, app, on what device, etc?
On my iPad, the Xfinity Connect app has an 'Edit' option when a folder is selectet that allows checking individual or all messages for 'Mark, Move or Delete'ing. I suspect the web mail access has a similar function. There may not be options to select 'Unread' only though. The web mail does have an option (top right, Sort) to allow you to sort the messages by 'Unread', which would at least group them together to start checking to delete.