if the link isn't tracked, how does that solve the problem of the user not having a client setup to handle mailto: protocol?
When a web-based MUA can recognize the link as using the mailto: scheme, it can open its own New Message window. As long as it looks like an http: link there's no way that'll ever happen. It's still not 100%, but greatly increases the coverage. (If you use an app that isn't registered as the mailto: handler and doesn't know to treat itself as the mailto: handler, you're still out of luck.)