Signing message, Set-Mailbox -UserSMimeCertificate

Hi

I have a question about how do I use the UserSMimeCertificate switch on a mailbox, example Set-Mailbox -Identity "Test 1" -UserSMimeCertificate

All our users have their certificate in AD so I can pick it up from the UserCertificateswitch.

Another question, if I set this switch on all our UserSMimeCertificate and they would like to sign a message (not encrypt) is it a requirement that the certificate is available on users workstation or can I get our exchange server to sign the message?

Br

Mikael