RIght now, if logged out, https://en.wikipedia.org/wiki/Special:EmailUser/Example displays a "Permission error" with no real explanation as to why (for new/annon users). Please add a similar message to that page:
You are not currently logged in.
Sorry, in order to email users you need to be logged in with a Wikipedia/Wikimedia/Mediawiki. Please log in (or create n account) and then try again.