Digital IDs, also known as digital certificates, help to validate your identity, and they can be used to sign important documents electronically. They can also be used to help protect messages by adding a unique code to the message called a digital signature. A digitally signed message proves to the recipient that you, not an imposter, signed the contents of the message, and that the contents haven’t been altered in transit.
To learn more about using digital IDs in Outlook, see Get a digital ID
To learn more about digitally signing documents, see Digital signatures and certificates
If you're just looking to add an "ink signature" to your Word document, see Insert a signature
A digital signature isn’t the same as a message signature, which is a customizable salutation. A digital signature adds unique code to a message which only comes from the digital ID held by the true sender.
Your organization may have its own policies and procedures for using digital IDs and certificates. See your network administrator for more information.
To find services that issue digital IDs for your use, or services that complement Office and use digital IDs, check out the following websites. It's up to you which one you choose, and others are available that are not in this list, but these are some certificate authorities (CAs) that are qualified to issue digital certificates.
Comodo To learn more and download the Free Email Certificate, visit the Comodo website.
GlobalSign To learn more about GlobalSign digital signatures, visit the GlobalSign website.
IdenTrust To learn more about IdenTrust's TrustID Secure Email Certificates, visit the IdenTrust website.