why is macmail sending smime.p7m

Are you encrypting the message?

Perhaps it is not Mail sending it but some server on the way or the recipient's email client.

Open Sent folder and find that message. Go to View > Message > Raw source.

Look for a block of random characters. That is the attachment. Before it will be the description that would look something like this:

--Apple-Mail=_D4D57B6A-3841-4EBC-B0D3-FA7EA3207BD4
Content-Disposition: attachment;
	filename="some file.txt"
Content-Type: application/octet-stream;
	name="some file.txt"
Content-Transfer-Encoding: quoted-printable

If the Content-Type does not say smile.p7m, it was not Mail that converted it.