I guess a good example of a moat in consumer email could be the fact that once you have the user, their email address is forever tied to your service. They can

I guess a good example of a moat in consumer email could be the fact that once you have the user, their email address is forever tied to your service. They can try to forward emails or fetch them, but they can't retain that email address on your domain without still using you (at least to some extent). Sender authentication then means they likely need to use you to send emails. That's a fairly effective barrier to switching for many people, as they won't want to lose their email address now, or need to go through every site they ever signed up with and change it.