When I first had a Microsoft Windows personal computer I put it on a computer desk in the lounge and bought a typical office chair with five castors. Over the next few years wheeling that chair around the desk area with my weight on it seriously damaged the carpet in that location. So I had to find a way to prevent office chair castors damaging carpets. See this discussion on the subject.
This article covers some of the issues involved when using a Digital Signature. Each Digital Signature is generated from a Digital Certificate (otherwise known as a Digital ID) issued by a certifying authority.
All examples given are created using Microsoft Outlook 2010 on a PC using Microsoft Windows 10, but the principles apply to other versions of Outlook and other email clients.