Ok, I can only assume that it's to do with SMF updates changing something. The way email is displayed is handled inside the theme, and the SMF 2.0 RMRK theme is based on
another 2.0 theme of mine, which doesn't display it. The only difference (besides obviously colours / containers) is that I haven't updated that forum's version.
For now I nuked that part of the theme. Still investigating what caused that
To those who were emailed, did you have the "Allow users to email me" setting enabled? It's on by default I think.