Version:           1.1 (using KDE 3.4.0 Level "b" , SUSE 9.3)
Compiler:          gcc version 3.3.5 20050117 (prerelease) (SUSE Linux)
OS:                Linux (i686) release 2.6.11-rc1

RfC 2045, page 4:

All of the header fields defined in this document are subject to the
general syntactic rules for header fields specified in RFC 822.  In
particular, all of these header fields except for Content-Disposition
can include RFC 822 comments, which have no semantic content and
should be ignored during MIME processing.

RfC 822, page 10:

comment     =  "(" *(ctext / quoted-pair / comment) ")"

Test case:

---
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable (comment)

=C3=B6 =C3=A4 =C3=BC=20
	bla=09
=09
=20
---

The body is displayed "raw" by KMail/Kontact. Removing the comment brings back proper display of the mail.