FastMail isn't scanning your emails to generate a profile of you in order to serve you highly targeted advertising all over the web.
Dick Cheney turns to a minion and says "I want to fuck up dombili's life. Get me his inbox." You think the minion is going to say "sorry, boss, he uses fastmail" and that's going to be the end of it?
If you prove one of my servers is infected I'd be wrong to assume it's only got one backdoor.