If they control the domain, they can see all incoming mail delivery attempts to sniff for addresses that were used. They’d still have to know the domain of the email address for the login they were attacking, which might not be super useful if they’re going after a certain login. But, going the other direction would be more fruitful: buy a domain, dump all incoming mail into a catch-all box, and start looking for bank alert emails or other periodic/promo emails. You might find services that just use email addresses for a login name, or ones that have a “forgot username” feature that only uses email for recovery. Multi-factor auth spread across multiple services (email, SMS, authenticator codes…) would help mitigate significantly by making them also have to take over a phone number or get an old device. Not impossible, but then you’re making them work harder for it, and when good account recovery services heavily mask the available targets, it makes it harder to know what else to acquire (e.g., a specific phone number) even if they get as far as full email domain control.