base64 encoded emails are not decoded

Are you sure it's not the mail (can you attach one of them)?

It seems unlikely that it's the mail. It only stopped displaying it after the upgrade to karmic and it still displays correctly with other clients. I've removed the routing information and the header contents.....

It's probably fixed in 3.7.3. Can you try the Claws Mail PPA ?

Yes - 3.7.3 fixes it.

This bug was fixed in the package claws-mail - 3.7.4-1ubuntu1

---------------
claws-mail (3.7.4-1ubuntu1) lucid; urgency=low

  * Merge from Debian unstable (LP: #458525), remaining changes:
    - debian/patches/50disable_can_change_accels_by_default:
      the gtk_can_change_accels pref which allows to change keyboard shortcuts
      from Gtk+ menus default to false for consistence with the GNOME default
      and other UME apps.
  * Refreshed and DEP3-tagged the patch.
  * New upstream version:
    - Looks better with dark themes (LP: #466835)
    - Handles base64-encoded e-mails correctly (LP: #447264)
    - Fixes a crash with IMAP (LP: #480748)

claws-mail (3.7.4-1) unstable; urgency=low

  * New upstream release
  - Fixes crash when copying mail via IMAP (Closes: #554781)
  - Fixes attempt to connect all accounts on exit (Closes: #552295)
  * debian/control
  - Fix typo in -doc description (Closes: #557456)

claws-mail (3.7.3-1) unstable; urgency=low

  * New upstream release
  - Fixes 'no refresh of lists after upgrade' (Closes: #548689)
  - Fixes 'entries not removed switching folder' (Closes: #548362)
  - Fixes 'bashism in /bin/sh script' (Closes: #530057)
  - Includes 'script for convert arbitrary Evolution mail folders
    (mbox format + subfolders)' (Closes: #461435)
  - Fixed previously 'parsing CSS in HTML causes quit' (Closes: #522424)
  * debian/README.source
  - Added file (thanks lintian!)
  * debian/control
  - Use the newer libreadline6-dev (thanks Matthias Klose for notifying)

claws-mail (3.7.2-2) unstable; urgency=low

  * debian/control, debian/rules, debian/claws-mail-dillo-viewer.manpages,
    debian/manpages/claws-mail-dillo-viewer.xml, debian/manpages/Makefile
  - Acknowledge NMU and remove dillo plugin: has been too much time
    since last time claws-mail was in testing and seems dillo2 won't be
    ready for the next release (Closes: #510597)
  * debian/rules
  - Use system config.{sub|guess} in preparation of the new source
    package format "3.0 (quilt)" (Closes: #538611)
  * debian/control
  - Update Standards-Version to 3.8.3 (no other changes required)
  - Added Homepage (incredibly missing until now!)
  * debian/patches/*.patch
  - Added patch descriptions
 -- Ilya Barygin <email address hidden> Thu, 21 Jan 2010 22:53:30 +0300

I have put a suggested fix in
 http://www.thewildbeast.co.uk/claws-mail/bugzilla/show_bug.cgi?id=2076#c12
But I have no idea if it gets any attention.
-- yotam