Message: Can not sign or encrypt message because certificate is n.

[City of Freer]

I receive a message when I try to send an email. The message reads: Can not
sign or encrypt message because certificate is not valid. How do I validate
my certificate? When I go to my certificate it shows that it is valid. I
need to get this messag to stop poping up. I think this is preventing my
emails from being sent. HELP!