This will be quick. I use GMail but it lacks the native ability to sign and encrypt email. I did some looking around for desktop email clients for Mac OS X that support S/MIME.
S/MIME support is provided by a plugin. Follow the instructions after going to the link, you’ll ultimately download a zip file.
Open Airmail, click on the
Window menu and select
Show Plugins Folder....
Extract the S/MIME plugin zip file and copy the
.bundle file into the plugins
Click on the
Window menu and choose
Plugins and click
You should be all set to send signed and encrypted email.
note: Comodo offers a free email certificate.