Article:
  How to Set Up Encrypted Mail on Mac OS X
Subject:   Mail security icons inoperable
Date:   2006-08-26 11:41:29
From:   PCheese
Response to: Mail security icons inoperable

Do you have multiple accounts in Mail? Remember, the certificate is only valid for a specific email address. If the Accounts popup shows an email address other than the one that corresponds to the certificate, you will not be able to sign the email. Of course, you can log in to the Thawte website, add a new email address, and get a new certificate for it so you can sign email from each of your accounts.