We currently have a complete copy of all user data in a secondary (non-US) data centre. In the event of a loss of our US-based servers, we would get this secondary copy up and running as quick as possible (likely in a reduced capacity) while sourcing new equipment and getting a new primary centre up as quickly as possible.
This would be a catastrophic event, no doubt about it, and there would be significant disruption for our users. But it wouldn't mean the end of FastMail.
We also have a second (1-2 week old at this point) backup set of most users' data sitting in boxes on my loungeroom floor. Encrypted of course. It came back in my suitcases from New York a week ago.
Bootstrapping from that would be significantly more painful though, and a lot more "gappy".
This would be a catastrophic event, no doubt about it, and there would be significant disruption for our users. But it wouldn't mean the end of FastMail.