Email notification for specific emails containing "n words" if n > 500

Solution 1:

If using Gmail or Google Apps (Google for Work or G Suite), filters may be of help. You can make specific email go to a specific folder on arrival, bypassing the inbox.

There is a discussion here on Gmail filters and wildcards that may point you in the right direction for creating the filter. Questions about Gmail will have better (and non-off-topic) answers there.

The problem with this solution is that you won't get notified.

I can think of two ways to get actual notifications from those specific search terms: filter everything that does not match the criteria out of the inbox, so you'll only get notifications from incoming mail that matches, or adding an IFTTT recipe that notifies you when the criteria is fulfilled —this looks promising:

I'd go with "New email in inbox from search" trigger and then enter your search terms. Here is a guide from Google on search operators that allow you to construct a custom search, OR probably, but you'll have to experiment the search syntax that gives you the desired result.