I am a person who doesn't have any brand loyalty. If there's something else that's better or has the same features at the same cost, I will go for it. That being said, Fastmail has been great. Besides the unlimited domains and masked email features, I never had an issue with my emails ending up in someone else's spam folder. This is crucial to me not to lose a client or a job, or even government communications. Some might argue about security/privacy, but emails are never meant to be that medium for secure communications. Even with PGP you would still leak metadata, so if you are after security, don't use email. Other than that, I will be after reliability and ease of use features.