Tarpitting slows down email validation.

It’s a service on the mail server of the email address(es) we’re trying to validate that slows down responses to our inbound request. Tarpitting is a defence mechanism, originally created as a defence against inbound attacks on a network. The idea is to make a network an unattractive target for spamming, scanning and computer hackers. The server will respond, but very slowly. It’s called tarpitting because it’s sticky, slows things down and isn’t pleasant.

We’ll validate emails that are tarpitted via our one time lists service, because that service is designed to process over time and will obey instructions to revisit servers that ask us to ‘come back later’.

Our Realtime API is a more speed oriented service that carries out a fast, single check so that won’t revisit tarpitting servers and will instead return a result of ‘unverifiable’.