S/MIME certificate not found (Windows)

I cannot encrypt mails with S/MIME certificates. The public certificate ist imported and visible under “certificates and keys” in the settings for the respective e-mail recipient. Yet eM client will always complain that no encryption technology can be used because valid public S/MIME or PGP keys are necessary for all recipients.
PGP encryption works fine though.

Do you have public S/MIME certificates for all of the recipients of the message?

What about your own private S/MIME certificate?