A greylist is a temporary mail deferral that asks the sending server to retry later before the message is accepted for delivery.
The receiving side remembers the sender, recipient, and sending address and accepts the same attempt after a short wait.
How it works
Many unwanted senders transmit once and never return, while a normal mail server is built to queue the message and try again.
The deferral is therefore a test of retry behavior, not a judgment of the words inside the message.
Greylisting is not the bulk mail a person once subscribed to, and it is not a permanent refusal of a domain.
Legitimate mail can arrive a few minutes late, so the control needs a timeout that is long enough to matter and short enough to keep business mail usable.
Concrete examples
- A company mail gateway answers an unknown sender with a temporary failure, then accepts the same message when the sender retries ten minutes later.
- A newsletter platform that retries on schedule passes the deferral, while a one shot spam script does not come back.
- A traveling employee sees a customer message arrive slightly late because the sender had never written that mailbox before.
Protective measures
- Use a temporary deferral code so honest servers know they should retry rather than treat the answer as a final block.
- Remember recent successful triplets so regular partners are not delayed on every single message.
- Watch for mail that never returns and for mail that is delayed past the time your business can accept.
- Do not rely on greylisting alone, because senders that retry on purpose will pass this test.
Related articles
Bulk mail that a person once accepted and rarely reads is explained in What is graymail in an ordinary electronic mailbox?.
The domain record that names the servers receiving mail is covered in What is an MX record in a company's domain name system?.
How an address earns trust or distrust from past behavior is described in What is IP reputation for one network address over time?.