How I delete massive amounts of email from Outlook

You can create an Outlook rule to delete any incoming email which satisfies your condition. And you can also apply the rule on your existing folders and delete any email which you have already received.

That still isn't ideal though. Right now I am trying to delete 260k of emails with that method and it's taking forever. By far, the fastest way to delete a large number of emails is to delete a whole inbox subfolder, but that only works if you have rules set up in advance so that you know it's ok to delete the whole folder. The only reason I'm not using that method now, is because I didn't have a rule set up for my current mass email problem.

For your particular problem, deleting the whole folder won't delete your associated rules. The rule will just have a step mentioning an unspecified folder. It'll be flagged as a rule with an error, so you'll have to go through and fix them. But, it'll probably still be faster than waiting for a rule delete.

If your outlook inbox is hosted on Outlook 365, you can set up a sweep rule and let the server clean up the mess in the background while your outlook is closed. I cleaned up an email mess of 100,000 emails by deleting emails from a particular sender older than 10 days old in about 3 hours

How to set up a sweep rule on outlook 365