How can you batch delete unopened emails?

Different ways to do this. Below are two ways, but you can be more elaborate, e.g. by setting up a rule, but if you make a mistake with the Mail rule, it might delete more than you want.

• Select and highlight the messages to delete, then just delete them. By holding down the Shift or Command keys, you can quickly select many messages this way.
• Perhaps more efficient, go to the mail provider's server, often this is via a web interface, and select the emails there (often as easy as checking a box), then instruct it to delete all checked messages. This will also remove them from your Mac if you are using Exchange, IMAP, GMAIL. For POP emails you have already downloaded, the first method needs to be used.

Make a new Smart Mailbox that only includes Unread Mails, click on that Box, select all delete or Move to Trash. :)