When accessing an Exchange 2003 server via IMAP, emails that were sent as text/plain (and ones that had no MIME encoding specified at all) get automatically converted to multipart/alternative with the original text/plain body and a text/html body. This is … stupid. It doesn't even bother to specify a monospaced font.
The new MIME part starts like this:
Content-Type: text/html;
charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
<HTML>
<HEAD>
<META HTTP-EQUIV=3D"Content-Type" CONTENT=3D"text/html; =
charset=3Diso-8859-1">
<META NAME=3D"Generator" CONTENT=3D"MS Exchange Server version =
6.5.7654.12">
<TITLE>{{subject}}</TITLE>
</HEAD>
<BODY>
<!-- Converted from text/plain format -->
<BR>
<P><FONT SIZE=3D2>{{body}}
(All the "3D" stuff is quoted-printable encoding for an equals sign; there's nothing wrong on that front, surprisingly.)
How can I make this stop?