I have a large amount of Bulk email coming in from Yahoo.

The general processing within mail is based on rules (Mail > Preferences > Rules) and rules can run scripts.

If you're looking to process stuff within the mail message, then I'd suggest a service like Zapier or maybe ifttt. If you want to roll your own, then there are various AppleScript examples around, such as this one or this one.

Per what's posted here, the Yahoo mail servers can be configured to manage the bulk folder for you.