Sometimes (I suspect after doing some action in message view panel, like opening an attachment) MailNews does something strange with the message encoding (falls into UTF ?) and it's impossible to set an apriopriate encoding (in my case, iso-8852-2). This happens when the Content-type/charset data is not set for the message (news sent by Outlook Express). In the message source window and when the message opened in a new window (doble-click) all looks fine.
can you reproduce this with a recent nightly?