My problem goes something like this:
A client receives an email sent by Exchange Server.
In the mail he has a formatted body with HTML with a couple of links that have
<a href='mailto:firstname.lastname@example.org?subject=Me&body=<b>ME</b>'>Mail me</a>
As you can see in RFC 2368, this is not possible at all:
The special hname "body" indicates that the associated hvalue is the body of the message. The "body" hname should contain the content for the first text/plain body part of the message. The mailto URL is primarily intended for generation of short text messages that are actually the content of automatic processing (such as "subscribe" messages for mailing lists), not general MIME bodies.