A guess is that people are sending HTML formatted messages to the list and
either the list server, a mail server somewhere in between, or individual's
e-mail clients are not able to recognize and display HTML.  So you see the
HTML code instead of the prettied-up text that the HTML is supposed to
render.

I use Outlook Express and I know that it defaults to HTML-formatted
messages.  Even when you set the default to "Plain Text" sometimes when
*replying* it still wants to run home to mama and format in HTML--you must
manually set it to "Plain Text" on a message-by-message basis.  (Unless
there is some hidden setting I have not found.)

So. . . if no server-related setting can be fiddled-with, I recommend folks
take special care to ensure they are sending "Plain Text" messages to the
list and see if that helps.

Just my $.02.

(I am sending this as "Plain Text."  Hope you can read it!)

-Michael