As of 2011, spammers have been using this technique for many years.
Delaying delivery also gives real-time blackhole lists and similar lists time to identify and flag the spam source.
After 4 hours the triplet will be expired, so delivery attempts will register anew.
When the greylister sees an attempt within the 25 minute - 4 hour window, the connecting host will be whitelisted for 36 days.
For example, a greylister can require a successful delivery attempt against a registered triplet to be no earlier than 25 minutes after registration and not later than 4 hours after it.
Repeated delivery attempts before the 25 minute period will be ignored with the same 4xx reply code.
Temporary errors are defined in the Simple Mail Transfer Protocol (SMTP) as 4xx reply codes: Fully capable SMTP implementations are expected to maintain queues for retrying message transmissions in such cases.
Because large senders often have a pool of machines that can send (and resend) email, IP addresses that have the most-significant 24 bits (/24) the same are treated as equivalent, or in some cases SPF records are used to determine the sending pool.Greylisting is particularly effective in many cases at weeding out misconfigured MTAs, and is gaining in popularity as a very effective anti-spam tool.It is likely that those MTAs that do not correctly handle greylisting will become less numerous as greylisting spreads.Rejecting a message after its content has been received allows the server to store a choice of headers and/or a hash of the message body.In addition to whitelisting good senders, a greylister can provide for exceptions.