Greylisting

Greylisting (described in RFC 6647) relies on a simple fact: legitimate mail servers retry after a temporary failure, while much spam software doesn't. When a server sees a new combination of sending IP, envelope sender and recipient, it answers with a temporary 4xx code (commonly 450 or 451) and records the attempt. If the same combination comes back after a minimum delay, the server accepts it and usually remembers the sender for a while.

RFC 5321 expects senders to keep retrying: it says the retry interval should generally be at least 30 minutes and the give-up time at least 4 to 5 days. So greylisted mail is delayed, not lost, as long as your sending server retries properly.

Greylisting causes the delays people notice on first contact with a new recipient domain. It also affects verification: a greylisting server gives a temporary answer to a mailbox check, so the result can't be confirmed on the first try. Don't treat these as bad addresses; they're undecided.

If a server greylists Skrap.ai's mailbox check and no definitive answer comes back, the result is Unknown, and Unknown results are free. You can re-check later in the free email verifier or via the API.