This works fine for me using trunk, but it does depend on your email client as well (I was using mail.app on Mac OS X).
There are lots of places that character encoding could fail along the way, PHP could be the culprit, so could the email server that the message is first sent through. Perhaps the mail client you are looking at it in doesn’t support or isn’t setup to view utf-8.
We have replaced the email sending code in Trunk, and that might be the difference too.
It would be good to see exactly what’s happening, can you post a link to your forum so we can try signing up to see what happens?
Thanks for your help on this and take attention on it.
The link of my forum listed as below.
http://www.plzon.com/forum/register.php
Thanks Sambauers