I've implemented "Tell a friend module".
The link of "mailto:" often makes wrong conversion of charactors in multi-byte-lang environment.
Even in single-byte-lang environment, it is impossible for a visitor without MUA to send "tell a friend".
This module works collaborately with a Smarty plugin modifier.xoops_tellafriend.php (Copy it into the plugin folder of Smarty before use this module)
All you have to do is editing your template set like this:
news_article.html
<a target="_top" href="<{$mail_link|xoops_tellafriend}>">