S/MIME, or Secure Multipurpose Internet Mail Extensions, is a standard for the signing and encryption of email. These certificates are the “user” version of SSL/TLS certificates you see used on websites. They authenticate and identify a user rather than a web server.
Usually, S/MIME certificates are going to run you a few bucks. I was able to find one certificate authority (CA) which provides Free S/MIME certificates valid for 365 days, Actalis, which is EU-based.
Do note, these certificates only verify your email address. If you want the certificate to verify/include additional details, such as your name/organization, you’ll have to pay. You won’t find a free CA to do this level of validation without payment since it requires human interaction.
More information on Actalis’ program is available here: https://www.actalis.it/en/certificates-for-secure-electronic-mail.aspx