Apple Mail - Turn off message signing

Background:

If you have a certificate on your Mac running OS X, Apple Mail may be automatically signing your email messages with it. This may cause undesired behavior:

• If it's unable to sign the message, you'll see the following error message:
  
• Recipients of your email may receive your messages with an attachment of type smime.p7s and this can cause issues with certain email programs being able to cleanly index and search your email

To turn off signing of messages:

1. Compose a new email message in Apple Mail.
2. Make sure that the option to sign messages is turned off.
   
   
   When turned off, the image should look like an 'X':