Definition
Greylisting is a spam filter that relies on how well-behaved mail servers work. When a server that uses it sees a new combination of sending IP, sender address and recipient, it answers with a temporary failure, usually a 4xx code such as 451 with a "try again later" message.
A proper mail server queues the message and retries after a few minutes. On the retry, the greylisting server recognises the combination and accepts it. Many spam tools never retry, so their mail never gets through.
The delay varies by setup. Some servers accept the retry after a minute or two, others wait longer.
Why it matters
For normal sending, greylisting mostly causes delays. The first email to a new contact may arrive several minutes late, and your platform may log a soft bounce or deferral before the retry succeeds. That is expected.
For verification, greylisting is a bigger obstacle. Verifiers ask the mail server about a mailbox without sending a message. A greylisting server answers that first question with "try later", so a single quick check cannot tell whether the mailbox exists. The verifier has to come back later, or report that the answer is unknown for now.
Tools that skip the retry and call these addresses valid or invalid are guessing.
How SimpleCold handles it
SimpleCold treats a temporary refusal as temporary. When a mail server is slow to answer, single and batch checks respond within about 12 seconds. If the mailbox check is still going, the result has status processing, and asking again within 30 minutes while it is pending does not use another credit. If the server never gives a clear answer, the result is Risky with the status no_response and the reason "The mail server would not confirm this address."
Developers can read more about the polling pattern in the handling processing results guide.
Related reading
- Unknown email verification results
- How SimpleCold handles slow mail servers
- Real-time email verification