In the Notepad it should be possible to sign&encrypt the text with S/MIME certificates.
This idea came up while discussing the merge request https://invent.kde.org/pim/kleopatra/-/merge_requests/176.
Currently, for S/MIME certificates the Notepad allows Sign or Encrypt, but Sign&Encrypt isn't possible. Therefore, the UI for selecting the signing and encryption certificates disables the selection of encryption certificates if the Sign checkbox is checked and vice versa. Technically, I see no reason for this. If a text in the Notepad can be signed with S/MIME and can be encrypted with S/MIME then it's also possible to first encrypt the text with S/MIME and then sign the (armored) result with S/MIME. Recipients will be able to reverse this easily.
I think it would be best if the combined signing and encryption would be handled by a GpgME job (probably best with an S/MIME-specific "copy" of QGpgMESignEncryptJob).