Tell users that mails needs to be validly signed to use keywords

We could think about if it would make sense, in terms of UX, to provide a helpful error message in case a mail wasn't signed, but it contained keywords.

Currently, that's not easy to do, because we only look for keywords if the mail was signed. I'm not sure how to tackle this, maybe via a "best effort" approach, via a superficial scan of the mail?

Any opinion?