There are 2 repositories under smime topic.
Simple API, Complex Emails (Jakarta Mail smtp wrapper)
Fork of OpenSSL 1.1.1 that includes prototype quantum-resistant algorithms and ciphersuites based on liboqs [OQS-OpenSSL 1.1.1 is NO LONGER SUPPORTED, please switch to OQS-Provider for OpenSSL 3]
.NET email library and proxy supporting IMAP, POP3, and SMTP with S/MIME and PGP.
AS2 Library for building and parsing Messages and MDNs
:closed_lock_with_key: :e-mail: Encrypts WordPress emails using OpenPGP or S/MIME with a familiar API.
Serialization of cryptographic data types
Great DANE Thunderbird extension
signing-milter enables you to s/mime sign an ordinary mail while passing a MTA. Base on https://signing-milter.org/ by Andreas Schulze
A poor mans tool to do S/MIME signatures in Python2 and 3.
Create TOTP secrets, encode in Google Authenticator compatible QR Code and send via S/MIME encrypted e-mail.
simple powershell module to get single or all user/contact certificates from an AD with all related information including metadata
View and compose SMIME encrypted messages in Outlook Web App on Linux
Cryptographic and MIME parsing library for Blackberry 10 based on OpenSSL
Spongy Castle - a repackage of Bouncy Castle for Android (which ships a crippled version of BC)
Nodemailer S/MIME Encryption via OpenSSL
Java Email Send Client with support for DKIM, S/MIME and PGP/GPG
Versandgigant WP Signed S/MIM Mails