To further protect users, ProtonMail utilizes only high strength ciphers for SSL and TLS.
We are posting our ciphersuite configurations for Postfix and Apache here for reference, we recommend all system administrators adopt these settings for better security.
Postfix ( main.cf )
smtpd_tls_mandatory_protocols = SSLv3, TLSv1
smtpd_tls_mandatory_ciphers = high
Apache ( ssl.conf )
SSLProtocol -ALL +SSLv3 +TLSv1
Finally, a quick note regarding TLS 1.2, we will transition to this protocol as soon as CentOS supports it.