S/MIME signature broken due to wrong MIME multipart header

When attempting to send a signed S/MIME message with evolution, the mail is sent as multipart/mixed instead of multipart/signed, thereby preventing signature validation. GPG signed messages are formatted and verified correctly. S/MIME signatures from other mailers are also verified correctly by Evolution itself.

Expected header:

Content-Type: multipart/signed; protocol="application/x-pkcs7-signature"; micalg="SHA1"; boundary="----=_NextPart_000_0097_01CC529D.8F3107C0"

Actual header:

Content-Type: multipart/mixed; boundary="=-y1IUcclJdzFtE471smGQ"

Ubuntu Release: 11.04

evolution:
  Installed: 2.32.2-0ubuntu7
  Candidate: 2.32.2-0ubuntu7
  Version table:
 *** 2.32.2-0ubuntu7 0
        500 http://de.archive.ubuntu.com/ubuntu/ natty/main amd64 Packages
        100 /var/lib/dpkg/status