There are millions of active domains used for email purposes, so naturally, it is impossible to provide a meaningful list of domains that cannot be verified. That said, we can verify the most common consumer ISPs such as Yahoo, Hotmail, AOL, Gmail, Comcast, Yandex, Orange, Voila etc. However, it’s important to note that with batch verification, some ISPs limit the number of emails that can be verified in one go, and where this happens we will return a result of “Unknown”. This should only affect a small minority of emails in your list.