Depending on how many you are talking about, there are different ways to approach mass deletions. If you want to clear out a folder, right-click on it and you will see the option to empty it. Or, you could do a search for a date range, then delete all the search results. Or, you can click on one message to select it, then hold down the Shift key and scroll down until you get to the last one and click on that to select everything in between, and delete them all.
How do I delete 4 years of comcast/xfinity emails at once
Assuming they are all in the same folder, you can right-click on that folder and then select "Delete all lmessages" from the menu that appears.
To delete old emails by date - oldest to newest
Sort file folder - ascending order by date
If you don't scroll down you will be able to delete only 50 emails at a time
Scroll down (you can delete 1,000;s) to approximately date you want to delete
Highlight by opening an email
Do a contol - A - this highlights emails and will return the number of emails hightlighted
Delete Hightlighted emails with icon or Right click and delete
I have not figured out how to contol the exact number and date of the newest email that is deleted.
Deleting groups of Comcast emails at a time
In addition to the scrolling method described above, you could run a date range search and then delete the results. Use a range like 01/01/2019-06/31/2019. The search field is on top of the folders pane.