OS X Lion: Mail doesn’t send certificate-signed message; Compose window cannot be closed

When attempting to send a message in Mail that has been signed by a trusted certificate, a message may briefly appear that states: "Unable to sign message You don’t have a trusted certificate in your keychain that matches the email address (sender’s email address). Without a certificate, you can’t sign messages sent from this address." The message quickly disappears, and the Compose window reappears but cannot be closed.