We've made a small change to the configuration on our servers that updates the SSL certificate we were using from one that was just for www.fastmail.fm to a wildcard *.fastmail.fm certificate. For users, there should be no visible change, and everything should just continue to work as normal. However the last time we made a change like this, there were some very old email clients/browsers that had problems, so this is just to let people know that might suddenly receive a new warning message, nothing is wrong, it's just that your email client/browser is very old and should be updated.