The question here is, of course: what makes a post relevant? I’d distinguish between two types of answers:
- What the community finds useful.
- What the individual user finds useful.
For type 1 relevance, a good operationalization could be “most liked posts” or “most liked topics” and while I don’t know the exact formula behind the composition of summary/digest emails, I’m sure that likes weigh in rather heavily into the equation. So if people are getting, say, weekly digests, but still feel they are unable to keep up, they are either asking the impossible (“please inject relevant knowledge into my brain without me having to read anything”) or they are looking for type 2 relevance.
For type 2 relevance, watching or tracking tags/categories has already been mentioned, but the safest and most flexible mechanism is probably saved searches. Install the plugin, write a help-post about how to use it and direct people who send you those PMs to that topic.
That said, there are probably ways of improving the saved searches plugin for your use case (and probably of broader relevance). For example, wouldn’t it be great if you could specify additional criteria that need to be met before you are notified about a post, e.g. must contain searchterm and have a minimum of 3 likes? Or must contain search term and be liked by a staff member. Or … and be tagged with X.