In the very beginning (almost 10 years ago now!), FastMail only had an SSL certificate for www.fastmail.fm, so to make IMAP/POP/SMTP over SSL work, people had to setup their email software to use www.fastmail.fm as the server name.
Now this hasn't been the case for almost 8 years, and for almost 8 years we've advertised that the correct IMAP/POP/SMTP hostname is mail.messagingengine.com, but for the whole time we've still supported IMAP/POP/SMTP via www.fastmail.fm for backwards compatibility.
To help cleanup some annoying old and error prone configuration files, we're soon going to start enforcing the correct hostnames for the different services. This means that in week or two, people using the incorrect hostnames for services will start getting errors when they try and connect.
Before we do this, we'll be trying to track down everyone using the wrong hostnames (based on logins to the wrong IP addresses), and alert them that their configuration needs updating. So in the next few days if we detect that you appear to be using the wrong hostnames, you will receive an email instructing you to update your email software configuration.
In the future, the correct hostnames and ports for all our different services will be the ones as described here: http://www.fastmail.fm/help/remote_email_access_server_names_and_ports.html
The port numbers listed above are for (plain/ssl) connections. You should double check your software to ensure that you are correctly using these hostnames and port numbers.