I want to digitally sign my e-mail
To digitally sign e-mail you need to request a digital signature.
The signature prevents that someone else can change the content unseen, and it enables the recipient to verify the identity of the sender. The certificate can be installed in mail clients that support the use of S/MIME. (S/MIME is the standard for secure emailing with certificates). The certificate is not trusted by default in Adobe Reader and is therefore not suitable for PDF signing.
Each user that whishes to sign needs an email certificate. The email address in the certificate has to match the email address of the user.