install stunnel. It is used for tunneling with OpenSsl.

see Postfix book on how to configure and use it.

Get key from OpenSSL, see stunnel -V for path to stunnel.pem:

$ cd /usr/lib/ssl/misc
$ cat newcert.pem key.pem >/etc/ssl/certs/stunnel.pem
$ chmod 400 stunnel.pem