You can not add duplicate domains, so if customer a added domain1.tld as mail domain, customer 2 cannot add it. So if thats all you want, then its already implemented. But it wont solve the phising problems as these problems arise when someone adds a domain which is not used on your server but might be used by other customers to send email to.
Okey, but this is so in every host, so this is not a defect and this is not the problem but the problem is that you can make :
And it's work perfectly with every domain, THIS is what i am talking about