One-stop resource on how to effectively disable SSLv3 in major web browsers as well as in web, mail and other servers that may still be using it.
Put the following lines in your configuration file, or modify existing:
smtpd_tls_mandatory_protocols=!SSLv2,!SSLv3 smtp_tls_mandatory_protocols=!SSLv2,!SSLv3 smtpd_tls_protocols=!SSLv2,!SSLv3 smtp_tls_protocols=!SSLv2,!SSLv3
Then restart the server (in Ubuntu: sudo service postfix restart).