TODO

* Find out why sometimes the new body cannot set to a MAPI object. In
  this case the body is empty but the W32 API said it was correctly set.
  This might be due to the length of the object.  HrGetOneProp has
  such limitations and thus it would be reasonable to assume that the
  same holds true for HrSetOneProp.  We might want to use OpenProperty
  for longer texts.

* Allow for symmetric encryption.

* Please consider adding a few more options like key signing rules
  like Enigmail has.

* We should not write a decrypted file without user consent.  A
  possible solution in attach-file-events.c is to keep track of
  presented file names and decrypt them only on OpenSzFile.  Need to
  find some documentation first.