I've abstracted some headers from the distributed list which may be
relevant (as you can tell, I'm using Eudora 5.2.0+

<Begin Forward>
MIME-Version: 1.0
Content-Type: text/plain; charset="iso-8859-1"
Precedence: list
Content-Transfer-Encoding: 8bit
X-MIME-Autoconverted: from quoted-printable to 8bit by f07n01.cac.psu.edu
id OAA307966

This list's definition headers:

*  Language= NoHTML
*  Attachments= No,Filter

Under LISTSERV 1.8e, I believe the noHTML removes the MIME section:

Content-Type: multipart/alternative;

that is the "styled" and only transmits the only / remaining "plain"

Content-Type: text/plain