Howdy, and welcome to the forum!
There are a few settings that you might want to check:
- Check that you are trying to sign the message, by pressing the Delivery Options button while composing the message and making sure that the "Sign" check-box is checked.
- Make sure that you are sending MIME-formatted mail, not Notes-formatted mail. The recipient should either have "Prefers MIME" set in his or her person record, or you should be sending to an internet-style address, and have "MIME Format" set in the "Format for messages addressed to internet addresses" field of the Mail tab of your current location record.
- Check to make sure that the certificate associated with the private key on the smartcard is your default S/MIME signing cert. Open the Your Identity//Your Certificates pane in the User Security Dialog, select the same internet certificate (there should be a small rectangle in the upper-left corner of the icon indicating that the private key is on a smartcard), and click on the "Advanced Details" button. The check-box next to "Use this certificate as your default signing cert" should be selected.
Give that a try, and please let us know if it works.
Good luck,
dave