Copy mailnotify.tmpl to mailnotify.text.tmpl and remove the lines shown below from the new file. Also, in mailnotify.pl, change

$text = &TWiki::Store::readTemplate( "mailnotify" )

$text = &TWiki::Store::readTemplate( "mailnotify.text" )

Lines to Remove:

MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="------------2D594AE113AD25493C2C7246"

This is a multi-part message in MIME format.
--------------2D594AE113AD25493C2C7246
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit


--------------2D594AE113AD25493C2C7246
Content-Type: text/html; charset=us-ascii; name="CodevChanges.html"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline; filename="CodevChanges.html"
Content-Base: "https://www.twiki.org/cgi-bin/view/Codev/WebHome"

%EMAILBODY%

--------------2D594AE113AD25493C2C7246--

-- RichardEllery - 05 Nov 2001

-- MartinCleaver - 12 Jul 2003