I've never tried it but I'd bet you could use instead use a key derived from the master somehow to salt a hash for all the stems too. I think that'd let you still do the search and possibly have a good index for it still but it'd add some hurdles to the whole thing so that you couldn't make a lot of good guesses about the actual contents of the emails. It would still leak information a out related emails (say there's a thread discussing a unique term in it), but it'd still be better than otherwise.
In theory you could HMAC all the stems, but at that point you really need to ask yourself what threat model you're trying to protect against. E.g. keeping only the stems is more than enough to prevent competitors from learning anything useful about your business if your database gets compromised, but using an HMAC probably isn't going to significantly slow down a state-level actor who has a warrant or zero day for your web servers. So on the balance I think the technical debt that would be introduced would likely make the overall system less secure, at least for most use cases.
