it clearly depends on client settings. in the company i work for we had a similar question. we ended up using ae
for ä
and so on.
we faced the problem that some clients running ms outlook 2003 were reading it as utf-8 while other clients running outlook 2003 were reading it as a defined ansi table.
another issue is involved in the usage of other mail clients. we also have a bring your own device program where employees could install linux or have mac books. they are using mozilla thunderbird, mutt and so on. other clients have set gmail as their prefered mailto: handler etc. so a global change of the outlook settings through a registry edit as in Rfilip's answer would not apply to us.
anyways.. you can only define the text body of the mail. sadly its impossible to define a html body through a mailto:
link. if this would work, encoding could be defined through the body.
there also is no such a flag to set up the prefered encoding inside the mailto:
link.
answer to this question: its impossible to detect or set the mail clients encoding so you should switch to characters that would work in every ansi table.
as alternative you could provide two links. one encoded with utf-8 with a note that, if it will display weird, the user might use the second mailto link.
or.. you provide one mailto link aswell as a drop down box to select encoding. (including a brief description of what encoding is as hover ontop of the dropdown)