Glossary · Email
Greylisting
A spam defence that temporarily rejects mail from unfamiliar senders, expecting legitimate servers to retry.
A greylisting server answers the first delivery attempt from an unknown sender with a temporary failure. Real mail servers queue the message and try again minutes later, at which point it is accepted. Most bulk spam software never retries, so the technique filters a lot of noise cheaply.
It interferes with live verification. An SMTP probe gets a temporary rejection that means neither yes nor no about the mailbox: it means come back later.
The correct behaviour is to report the outcome as unknown rather than guessing. Treating a greylist response as a hard failure would mark valid, active mailboxes as dead.