The few times I've needed support, Fastmail has responded nearly immediately with the exact info I needed. So Moz would need to demonstrate excellence in custom

The few times I've needed support, Fastmail has responded nearly immediately with the exact info I needed. So Moz would need to demonstrate excellence in customer service before I would consider any migration. (I know that doesn't directly answer your question, but it does articulate a necessary pre-condition, and one that is hard for businesses entering a new line of service to deliver--although not impossible, of course.)