There are services readily available as you search on Google.
However it is highly recommended to try Best Database Provider by buying small sample sizes, and then take them through an email list verifier.
An email list verifier goes through your list, email by email, performs deep checks on each of them, and provides you with a report back regarding which emails are valid, and which ones are of not only of no use, but also damaging your IP reputation. What does a low IP reputation mean? Well, your email campaigns end up in the Spam folder instead of Inbox for starters. Then, the ISP can block you for further sending out campaigns.
The emails that an email list verifier is checking for are:
-invalid/incorrect ones due to typos;
Checking your list with an email list verifier before sending the campaigns out is no brainer, considering the damage a high amount of bounces can do to your business.