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

What happened

HCL 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.