Maybe you should put up much more strict rules regarding reviews if you are going to majorly relay on them. Having multiple emails even if they are not throw away emails is quite easy. For example there are services that provide temporary verifications for Gmail account using very many phone numbers. Some charge $0.1 per verification number. So someone can even have over 100 verified Gmail accounts for just $10

And trust me, scammers or exchanges that fake volume can do anything to keep up their reviews faked too. It will be an even easier job for them as compared to faking volume. Just look at how messed up trustpilot is.