E-mail signing or digital ID's are used to protect and validate communication. It encrypts e-mail messages, only allowing the intended recipient to read it.
There are personal signing certificates which can validate an individual (email@example.com). This type of digital identification will only be issued after a verification of the identity of the person placing the request.
There are also certificates that are issued on a department- or organisation level (firstname.lastname@example.org). For these certificates, the identity of the company or organisation is verified.
After a successful validation, these certificates can be used to sign e-mail (S/MIME), Microsoft Office documents, or other electronic files.