You and signer have no certificates in common; signer cannot be assumed to be trustworthy.

What happened

IBM® Notes® can't verify the digital signature on a mail message.

What you can do

You and the sender have no certificates in common, therefore you do not implicitly trust the sender's certificates. To view all of the Notes and Internet certificates that you trust and don't trust (and alter those trust decisions) for specific people, services, or certificate authorities, choose File > Security > User Security (Macintosh OS X users: Notes > Security > User Security), and then click Identity of Others > People, Services or Authorities.

For information on determining trust for certificates, see Certificates for people or services and Finding Notes and Internet certificates you trust.