I know that fastmail will be in business in two years, but I'm not so sure about you.
If you offered some sort of auto-backup option, so that even if you went down I could take my mail elsewhere, that would be more compelling.
The infrastructure I'm running on really is pretty cheap. The biggest expense by far are the databases, which run about $250/month for two. If I had to pay out of pocket to support even just a few users for a year, I'd do it.
Anyway I think the best paranoid option (no matter what mailhost you're using) is to set up automatic forwarding to a backup address. Or you can use imapsync [0] from time to time, which is a bit finnicky but gets the job done pretty well. (I actually might try putting up a quick web interface for imapsync sooner or later to cover import/export use cases.)
Misses sent mail though...
> imapsync
If the emails are deleted on the server doesn't the client just delete them too?
Is there any chance FastMail will implement this anytime soon?