Here's a sample mailto, from my site, that will also fill in the subject field.
<a href="mailto:aciinquiry@nccelectronics.com?subject=Web: ACI Inquiry; ">ACI</a>
After the question mark you could use body=body text here\n\nWith this on a new line
To use both subject and body, they need to be seperated by a ? (i think the seperator is a question mark, i'm not sure)
Regardless of whether alone or combined with a subject i have had problems with the body element.