The cause could have been security software removing your email address from the environment variable.
I know at least one of the Symantec identity protect applications could be very aggressive in removing personal data such as email addresses from certain areas in my computer. When I told it to protect my email address it caused all sorts of grief. I had to tone it down or turn it off, I can't remember which. Anyway, that functionality has since been removed from Norton 360, which I use now.
Of course, having your email address in an environment variable may expose it quite a bit when you are online. I'm not sure if web sites can see environment variables, but if they look, it is possible. Perhaps check that out.
Also, sometimes if some information is not available via a Registry setting, it can be placed into an environment variable as an alternative. Perhaps check with the author of mc2xml whether this is the case, and how to avoid using an environment variable.