What’s a “greylist test”?

Greylisting is a spam defense where servers temporarily reject first delivery attempts, expecting legitimate senders to retry. Greylist testing checks whether validation attempts are being greylisted, which can cause false invalid results.

When validation hits a greylisting server, the initial rejection might be misinterpreted as "invalid address" when it's actually "try again later." Quality validation tools recognize greylisting responses and handle them appropriately through retries.

This matters for validation accuracy. If your validation provider doesn't handle greylisting, addresses at greylisting domains may be incorrectly marked invalid. Ask providers how they handle greylisting when evaluating validation services.