A tarpit (also known as Teergrube, the German word for tarpit (German pronunciation: [ˈteːɐ̯ˌɡʁuːbə])) is a service on a computer system (usually a server) that purposely delays incoming connections. The technique was developed as a defence against a computer worm, and the idea is that network abuses such as spamming or broad scanning are less effective, and therefore less attractive, if they take too long. The concept is analogous with a tar pit, in which animals can get bogged down and slowly sink under the surface, like in a swamp.

This passage was taken from https://en.wikipedia.org/wiki/Tarpit_(networking)

Tarpitting can significantly slow down the email verification process and as a result only our Bulk List verification service will verify such addresses. To ensure a speedy response time, our Realtime API product will return such addresses as 'Unverifiable'.