Please see http://archives.gentoo.org/gentoo-user-cs/ or http://archives.gentoo.org/gentoo-user-cs/msg_00761.xml. The utf-8 bytes are rendered as if they were from some legacy encoding.
This issue is primary a Mhonarc bug, I've upgraded Mhonarc to 2.6.15 from 2.6.10 to fix it. http://www.mhonarc.org/archive/html/mhonarc-dev/2004-12/msg00000.html In addition to this, the content-encoding http header is set wrong, I'm still tracking down how to change that.
Content encoding has been fixed for a while now, and this should be working on all of the lists.