Sources of Free S/MIME Certificates

Free certificates usable for S/MIME are available from:

• Actalis
• CAcert (CAcert is NOT one of the trusted authorities built-in to FireFox and ThunderBird. The connection is also untrusted)
• Comodo (They’ve changed their name to Sectigo and no longer offer free S/MIME certificates/)
• GlobalSign (No longer offer free certificates)
• InstantSSL (free certificate is now a 30 day trial)
• Secorio (Affiliated Partner of Sectigo (formerly called Comodo) that links to InstantSSL if you want a free certificate)
• StartCom (StartCom certificates have been revoked by Mozilla) [1]
• Wosign (WoSign certificates have been revoked by Mozilla)

Currently only Actalis seems to offer a free S/MIME certificate for personal use that is good for one year. Everybody else appears to offer a free certificate for personal use for only 30 days, or require you to buy one. It can also cost money to revoke a free certificate. [2]

Let’s Encrypt does not currently offer S/MIME certificates. See https://community.letsencrypt.org/t/s-mime-certificates/153 for a thread explaining why you can’t use their SSL/TLS certificates for S/MIME.