Help with sending email after importing certificate

I’ve been reading the forums and the knowledgebase, and have gotten to the point of having my certificate imported for Thunderbird. The email addresses match up on certificate and my email account, but when I try to send email I get this “Send Message Error”.

Sending of message failed.
Unable to sign message. Please check that the certificates specified in Mail & Newsgroups Account Settings for this mail account are valid and trusted.

When I view the certificate is says:
Could not verify this certificate for unknown reasons.

Can anybody help me?