I know in a post two years ago that somebody was already asking about mailto: links:
The answer was that they don’t work. I just encountered this in the calendar text and footer. I understand maybe in the first version of YCBM you maybe used a fresh library or implemented markdown yourselves but now Markdown is so ubiquitous that there are a lot of good libraries and it’s weird to have a semi-broken version of Markdown URLs (I can’t imagine Markdown would be very popular if it only supported certain types of links but not others!).
Great news, we have plans this summer to improve our Markdown to include mailto: links and opening links in a new browser tab. We will announce on the forum when those updates happen.