I've used several list verification tools in the past. But the biggest problem with a lot of them is that they don't do a perfect job. Result: valid addresses risked being unsubscribed (the way I like to treat email addresses that might bounce). If you want to stand out as an email marketeer (and avoid remarks like 'I just mailed that person at the address you mention as invalid'), ZeroBounce is the way to go. Plus: they offer several extra functionalities, what makes it a really complete toolset for email marketeers.