Greylisting is a technique of eliminating spam sent by virus-infested zombie PCs. In order to differentiate the two, a temporary error is simulated. A real email server will handle this correctly and make a second attempt to deliver the email. Zombies, on the other hand, will simply discard the spam emails.
If an email server is accepted by the greylist after a second delivery attempt, the server will be accepted for a certain period of time without any further delay.
Greylisting filters out a large amount of spam. However, this principle causes a delay in the first email sent between two contacts, depending on the repeat interval of the outgoing server (usually a few minutes).