Bulk email delete

From the Mail user guide:

Delete multiple emails

1. In the Mail app on your Mac, select messages or email conversations in the list.
2. All of the messages in a conversation will be deleted.
3. Click the Delete button in the Mail toolbar or press the Delete key.

If you delete a mailbox, all of the messages in the mailbox are deleted.

You can search for a set of messages to delete.

For example, you can search for messages dated August 1 through September 5, 2021 by entering “date:8/01/21-9/05/21” in the Mail search field. Or, search for messages from a specific person by entering a name in the search field. Then select and delete the found messages.