This article describes how private emails, which are written in Mozilla Thunderbird, can be signed before they are sent by the author. For this purpose, S/MIME (Secure/Multipurpose Internet Mail Extensions) is used, to encrypt and sign emails by X.509 standard.
In order to obtain such a certificate for signing emails, there are a few free CAs (Certification Authority). Here Comodo is used as CA, which offers this service free of charge for private use. Comodo confirms with a signature the authenticity and identity of the owner, for which the certificate is issued.