Created attachment 102784 [details] [review]
Strange that this one was never detected before ...

Created attachment 102785 [details] [review]
Two small fixes

Looks right.  Couple notes:

  1) The test for NULL is unnecessary since NULL is a valid GList.
     g_list_free() is designed to handle it.

  2) Minor simplification, unrelated to the memory-leak.  You could get rid of
     the unref_part() function and do this in the finalize function:

     g_list_foreach (multipart->parts, (GFunc) camel_object_unref, NULL);

Approving since your current patch correctly fixes the problem.  The stuff above is optional.

Committed