If you want to make a line break in the mail body to you could use this:
%0D%0A
its just a line feed and carriage return.
For white spaces you could use this:
%20
for more information look at RFC 2368
This is a little example:
<a href="mailto:paul.scheinast@yahoo.de?subject=criticism&body=Hello,%0D%0A%0D%0Ayour text.%0D%0A%0D%0Amfg%0D%0Auser">Mail to</a>
Try it:
Mail to