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Today we're rolling out SHA-256 certificates. We announced this last month, and you can read that post for more information about why this is necessary.
At the same time, we've disabled the RC4 cipher suite. RC4 has long been considered broken and the browser security community recently started actively discouraging its use. The SSL Labs test penalises it, and Chrome has started presenting a low-priority warning.
All this means that we're now get an A+ grade on the SSL Labs test, which is a good indicator that when it comes to our SSL/TLS configuration we're pretty much in step with current industry best-practice.
If, like most of our users, you use the web client in a modern web browser, you won't notice any difference. In older browsers and some IMAP/POP3/DAV/LDAP clients, you may start seeing disconnection problems if they don't know how to handle SHA-256 certificates or rely on RC4. In these cases you're encouraged to upgrade your clients and if necessary, contact your the author of your client for an update. In the meantime, you can use insecure.fastmail.com (web) and insecure.messagingengine.com (IMAP/POP/SMTP), both of support RC4 and have a SHA-1 certificate. As always, we highly discourage the use of these service names because they leave your data open to attack, and we may remove them in the future.