One thing that people often don't pay attention to is that fastmail.fm does a lot of Open Source work. One of the fastmail.fm crew, Bron Gondwana, has made extremely important contributions to the world's best, most scalable libre-software mail spool, Cyrus IMAP. He has commit rights upstream, and he has been a very active contributor and member of the community for a few years, now. fastmail.fm is a Cyrus IMAP shop, but they help other Cyrus IMAP users all the time in Cyrus the mailing lists, they have given us details about how to set up highly scalable and redundant multi-million-user setups many times now... And it is not rare to see Bron on the LKML (Linux Kernel ML) engaging the kernel developers to fix a kernel bug or two.
This is no publicity stunt, these guys are it: the email shop anyone who likes to tout around about the importance of FLOSS should vote for with their wallets.
Many thanks for these kind words - and thanks too for pointing out Bron's great work.
We're at a point now where every single piece of software we use, bar one, is open source. (The one exception is the RAID-monitoring tool that IBM requires for monitoring their RAID hardware.) Everything else, from programming languages, to operating systems, to system monitoring, email servers, web servers, anti-virus, anti-spam, and so on and so on, is all open source.
The reason we use open source for everything is that we can (and do) hack at the source to add features and fix bugs, we get direct access to the actual developers when we need help, we never have to rely on a third party to fix problems, and we get all the security benefits too.
For those who are interested, we have some more details on FastMail.FM's software (and hardware) infrastructure on our site.