I cannot find the reference, however, I am fairly certain that I have seen
documentation which advises that command messages should be in plain text
format.

Recent versions of some email clients allow you to set a flag in an address
book entry to specify whether messages for that address should be plain-text,
HTML, or mixed. If your email client software supports this feature, you may
want to consider creating an address book entry for your listserv server
command address, and flagging it as a "plain text" message recipient.

--
Paul Russell
Senior Systems Administrator
University of Notre Dame