I'm using S/MIME*, with gnus/epg and gpgsm from gnupg 2.0.29.  Generally
this works.  But I am finding that encrypted mail sent to some people
who use Microsoft Outlook cannot be read by them (and I haven't been
able to get any more useful information).  Some Outlook-using people can
receive simple encrypted text, but not encrpyted mail with attached
megabyte-class tarballs.  Mail sent to OS X Mail.app and Thunderbird
mostly works.

Has anyone else experienced these issues?  Are people convinced that the
messages produced by gnus/egp/gpgsm 100% comply with standards?

* Yes, I know that S/MIME has a lot of issues and that most here would
  tell me to use OpenPGP.  As you can see by this message, that's what I
  do in most situations.  But in this case my goal is to interoperate
  with people who theoretically have S/MIME as part of already-installed
  proprietary clients on machines they cannot control.  Lecturing them
  about Software Freedom, while enjoyable, is unlikely to be useful in
  the short term :-)