Hi,

On my free time I help developing an open-source MUA, Sylpheed-Claws.
Since I began working on it (about three years ago), we had
interoperability problems with Outlook Express (and Outlook). The main
problems being:

a) Outlook Express doesn't understand the PGP/Mime standard, which
  consists of a multipart MIME message, one part being the actual mail
  and the other containing the signature. This format is documented and
  standardized by the RFC 3156 (4 years old), which updates rfc 2015
  (9 years old).

b) Outlook Express doesn't understand the standard way of encoding
  structured header fields, such as MIME attachments filenames (MIME
  parameters, in a more general way). This standard is defined by RFC
  2231 (8 years old), whereas Outlook Express decodes and encodes
  attached filenames using the format described in RFC 2047.
  Unfortunately, the RFC 2047 applies to unstructured headers fields,
  like Subject or To.

References:
  http://www.faqs.org/rfcs/rfc3156.html
  http://www.faqs.org/rfcs/rfc2231.html
  http://www.faqs.org/rfcs/rfc2047.html 

I guess my question is: Does Microsoft plan on supporting these RFCs
one day? Any ETA by luck?

Thanks,