There is no straight answer to your question. The "mailto:" protocol envokes the user's MAPI program (Outlook, etc.) assuming it is configured to do so. The user is warned that his email address is going to get exposed, and he may cancel the email at that point.
Anyway, to get things in the body, you have to append things to the "mailto:" link:
< a href="mailto:email@example.com?body=This%20will%20be%20in%20the%20body">Mail me< /a>
It will probably look like crap...;-)