With a few others, I've been attempting to get S/MIME supported in
notmuch (which as you might or might not remember is using gmime
underneath). The signature verification part is working OK, but I've
gotten a bit bogged down trying to get decryption working.

What I have learned is that according RFC5751, smime message (parts)
look like

   Media type:  application/pkcs7-mime
   parameters:  any
   file suffix: any

   Media type:  multipart/signed
   parameters:  protocol="application/pkcs7-signature"
   file suffix: any

   Media type:  application/octet-stream
   parameters:  any
   file suffix: p7m, p7s, p7c, p7z

Unless I miss something, out of the box there is only support for
decrypting multipart/encrypted.  In particular the gmime tests for
S/MIME use this "container format" [1].

So I'm wondering if I'm confused about the RFC(s), or about GMIME, or is
one suppose to write analogs of g_mime_multipart_encrypted_decrypt for
other top level parts?

Cheers,

David

[1]: https://github.com/GNOME/gmime/blob/master/tests/test-smime.c