The following tags have self-explanatory href. In all cases the subject is 

è is not e
in the first exapmple it is the urlencoding of its rfc1342 representation 
=?iso-8859-1?Q?=E8?= is not e

good:
mailto:user@example.com?Subject=%3D%3Fiso-8859-1%3FQ%3F%3DE8%3F%3D%20%20is%20not%20e

bad:
mailto:user@example.com?Subject=è%20is%20not%20e

ugly:
mailto:user@example.com?Subject=%E8%20is%20not%20e

If you use the wrong tag on system that don't have iso-8859-1 compatible tables you get unexpected results. I tested on MacOS X and got Netscape working on good and bad, iCab almost ok with good, Internet Explorer setting Ë rather than è. Old browsers apparently send the "good" form literally, but when it arrives the subject is what was meant.