Sure, but I still don't know what process you are describing. mailto: is just a way of configuring a link on a web page to open an email client and a new email addressed to the address following "mailto". None of that has any influence on the encoding the email client is using. That's not up to you at all. If you are sending an HTML email, you can of course encode it any way you like.
Are you asking how to force people who are sending emails to use utf-8? That has nothing to do with web design as such.
So I'm still not clear on what you are asking. Do you see why?
EDIT: O, I see you are talking about fonts. Are you asking about how to set up your own email client?