ciphermail
Ciphermail email encryption gateway (S/MIME, OpenPGP, PDF encryption)
...Ciphermail Email Encryption Gateway has a built-in CA which can be used to issue X.509 certificates for internal and external users.
For additional security, keys can be securely generated and stored in a tamper proof Hardware Security Module (HSM).
The built-in Data Leak Prevention (DLP) module can be used to prevent certain information to leave the organization via email.
Ciphermail can be installed on most Linux and Unix based systems. Installation packages are available for Ubuntu, Debian, Red Hat/CentOS and OpenSUSE. A ready to run virtual appliance for VMware and Hyper-V is available.