...
# TLS/SSL protocols to use (avoid older SSL protocols)
ssl_protocols = !SSLv2  !SSLv3

# SSL ciphers to use
ssl_cipher_list = ALL:!ADH:!EXPORT:!SSLv2:!aNULL:!eNULL:RC4+RSA:+HIGH:-MEDIUM:-LOW
ssl_prefer_server_ciphers = yes

# SSL certificate
ssl=required
ssl_cert = </etc/pki/tls/certs/ssl-mail-cert.crt
ssl_key = </etc/pki/tls/private/ssl-mail-cert.key