Looking for suggestions and recommendations for a bulk e-mail validation / verification service, for on the order of 1/2 million records.
There are plenty of companies that can and do the work for you. ZoomInfo, Oceanos, Data.com, D&B, etc (a lot more than what I'm adding here). Personally for me, Oceanos has been my choice for a couple of places I worked at. It's all about scoping it out and seeing which vendor will give you the best bang for your buck. You definitely want to narrow it down to a few and then do some beta runs for each of them before deciding which one will best fit your business needs. It also depends on your company's policies around vendor selection. I worked somewhere that we had to pick from approved vendors.