I don't know how general this is, but when I send mail with this
installation:

   GNU Emacs 21.2.1 (sparc-sun-solaris2.8, X toolkit) of 2002-04-08 on hw-develop

the Content-Type: line specifies the content-type as "text".  My
reading of RFC 1521 indicates that this is not sufficient, and I have
found at least one MTA that incorrectly handles e-mail with this
Content-Type.  I draw your attention to:

     Note also that a subtype specification is MANDATORY. There are no
     default subtypes.

which appears in:

http://www.freesoft.org/CIE/RFC/1521/4.htm

The Content-Type: should be specified as "text/plain", or "text/plain;
charset=us-ascii", or the "Content-Type:" header field should not be
included at all (in which case the "text/plain; charset=us-ascii" is
assumed). 

A lovely thing about emacs' mail is that I can manually edit in the
Content-Type header and make it whatever I want, so that I can get
around my problem with the MTA I mentioned above.  But it looks like
emacs (or is it solaris's instantiation of sendmail?) is broken as far
as 1521 is concerned.

-- 
Mickey Rowe     (rowe@psych.ucsb.edu)