The signature application is unable to process e-mails that are signed, (a digital signature appended based on the contents of the e-mail to enable the recipient to tell if the email has been modified) or sealed/encrypted(contents of the e-mail are encrypted).
The application checks for e-mails that are signed or sealed and if they are then the application allows the e-mails to pass without adding a signature.
Signed Emails
Signed e-mails contain a field '$Signature'

Sealed Emails
Signed emails contain a field '$Seal'
