We had a bit of a laugh about that, because we don't really have any secrets. The fact is, it's just a ton of hard work - new spammer tricks, new interoperability woes (ask us about SSL negotiation with other people's MXes some time, but make sure you provide the alcohol), and just trying to get the best possible performance out of the hardware and the software so we can handle both the person with 500,000 emails in their Inbox who never moves anything and the person with 10,000 folders each with 5 emails in it, and everything in between - and provide a fast loading experience for every single user when they hit the site, even though there isn't space in memory to hold that many mailboxes cached.
When we started blogging the advent series last year (http://blog.fastmail.com/2014/12/01/fastmail-advent-2014/) I had a few support tickets asking "aren't you giving away all your secrets?"
We had a bit of a laugh about that, because we don't really have any secrets. The fact is, it's just a ton of hard work - new spammer tricks, new interoperability woes (ask us about SSL negotiation with other people's MXes some time, but make sure you provide the alcohol), and just trying to get the best possible performance out of the hardware and the software so we can handle both the person with 500,000 emails in their Inbox who never moves anything and the person with 10,000 folders each with 5 emails in it, and everything in between - and provide a fast loading experience for every single user when they hit the site, even though there isn't space in memory to hold that many mailboxes cached.