This is a serious issue that can leak user data. Unfortunately there’s no workaround – the only option we have is to disable SSL 3.0 on our servers entirely. We don’t like having to do this because we want our users to be able to use any client they choose to access their mail, but when there’s a security hole and no way to plug it we have no choice but to break things for some people in order to protect everyone.
Happily, this should not affect the majority of our users. The only significant browser to be affected is Internet Explorer 6 on Windows XP, which will now not be able to connect to www.fastmail.fm at all. Similar changes have been made to our IMAP, POP and other backend services, so you may also have connection issues with older mail clients.
If you are unable or unwilling to upgrade your client software at this time, you can use insecure.fastmail.fm (web) and insecure.messagingengine.com (IMAP/POP/SMTP), both of which support SSL 3.0. 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.
Update 16 Oct 2014 01:00 UTC: We’ve heard of at least two mail clients (Airmail and Windows Phone) that can receive but not send mail. Changing the outgoing settings to use port 587 instead of 465 has resolved the problem for some users. If you’re seeing similar problems, give that a try.