drop SSLv3, RC4 ciphers from SMTP port 25

Per http://googleappsupdates.blogspot.ro/2016/05/disabling-support-for-sslv3-and-rc4-for.html, Google is about to do the same.

fixes #611

CHANGELOG.md

@@ -7,6 +7,7 @@ In Development
 Mail:
 
 * Roundcube is updated to version 1.2.0.
+* SSLv3 and RC4 are now no longer supported in incoming and outgoing mail (SMTP port 25).
 
 v0.18c (June 2, 2016)
 ---------------------

security.md

@@ -101,7 +101,7 @@ Incoming Mail
 
 ### Encryption
 
-As discussed above, there is no way to require on-the-wire encryption of mail. When the box receives an incoming email (SMTP on port 25), it offers encryption (STARTTLS) but cannot require that senders use it because some senders may not support STARTTLS at all and other senders may support STARTTLS but not with the latest protocols/ciphers. To give senders the best chance at making use of encryption, the box offers protocols back to SSLv3 and ciphers with key lengths as low as 112 bits. Modern clients (senders) will make use of the 256-bit ciphers and Diffie-Hellman ciphers with a 2048-bit key for forward secrecy, however. ([source](setup/mail-postfix.sh))
+As discussed above, there is no way to require on-the-wire encryption of mail. When the box receives an incoming email (SMTP on port 25), it offers encryption (STARTTLS) but cannot require that senders use it because some senders may not support STARTTLS at all and other senders may support STARTTLS but not with the latest protocols/ciphers. To give senders the best chance at making use of encryption, the box offers protocols back to TLSv1 and ciphers with key lengths as low as 112 bits. Modern clients (senders) will make use of the 256-bit ciphers and Diffie-Hellman ciphers with a 2048-bit key for perfect forward secrecy, however. ([source](setup/mail-postfix.sh))
 
 ### DANE
 

setup/mail-postfix.sh

@@ -122,8 +122,9 @@ tools/editconf.py /etc/postfix/main.cf \
 	smtpd_tls_cert_file=$STORAGE_ROOT/ssl/ssl_certificate.pem \
 	smtpd_tls_key_file=$STORAGE_ROOT/ssl/ssl_private_key.pem \
 	smtpd_tls_dh1024_param_file=$STORAGE_ROOT/ssl/dh2048.pem \
+	smtpd_tls_protocols=\!SSLv2,\!SSLv3 \
 	smtpd_tls_ciphers=medium \
-	smtpd_tls_exclude_ciphers=aNULL \
+	smtpd_tls_exclude_ciphers=aNULL,RC4 \
 	smtpd_tls_received_header=yes
 
 # Prevent non-authenticated users from sending mail that requires being