Por assinaturas digitais, suponho que você esteja se referindo a e-mails assinados usando S / MIME (Extensões de E-mail Segura / Multipurpose Internet).
No site de suporte do Microsoft Office, esta link [1] deve responder sua pergunta.
Open the digitally signed message.
Look at the Signed By status line and note the e-mail address of the person who signed the message.
Check to see whether the signature is valid or invalid.
If the button on the Signed By status line appears similar to the following Signature button Button image , the signature is valid. For more information about the status of the signature, click the button.
If a red underline appears under the Signed By status line and if the button appears as an exclamation mark, the signature is invalid. For more information about the status of the signature, click the button.
[1] link