Definition
The email greylisting window is the time interval during which a receiving server uses greylisting to defer a message from an unfamiliar sender. When a server sees a first-time combination of sender, recipient, and sending IP, it temporarily rejects the message with a soft error, typically a 451 or 450 response. Legitimate sending servers retry after a short delay, while many spam-sending systems do not, so greylisting filters out low-effort spam at the cost of a small delay.
How It Works
Greylisting relies on the observation that legitimate SMTP servers are configured to retry delivery, whereas simple spam-sending bots often send once and move on. The receiving server records the sender, recipient, and IP triple, then rejects the first attempt with a temporary failure. The message is accepted only when the sending server retries after the required delay.
The window is defined by two settings:
- The initial deferral period, commonly around 300 seconds (five minutes), before a retry is accepted.
- The length of time the triple is remembered, after which the server may require a fresh deferral.
Senders with well-behaved retry logic experience only a short delay and are then allowlisted for a period, so the effect on delivery time is minimal. Senders with poor or missing retry behaviour may see mail delayed or dropped entirely.
Why It Matters for Senders
Greylisting is a legitimate anti-spam technique, not a form of blocking, but it interacts poorly with certain sending patterns. Bulk senders and transactional emails that are time-sensitive can be affected if the sending platform does not retry promptly or if retries originate from a different IP address, which resets the greylisting state.
Senders can minimise friction by:
- Using sending infrastructure with reliable, RFC-compliant retry behaviour.
- Retrying from a consistent IP address so the triple matches.
- Avoiding rapid-fire reconnection from rotating IPs, which can appear bot-like.
Greylisting is less common at large consumer providers today than it was a decade ago, but it remains in use at some corporate and smaller mail systems, so understanding the window remains relevant to deliverability.
Example
A small business sends an invoice to a client whose mail server uses greylisting. The first delivery attempt is deferred with a 451 response. The business's server retries seven minutes later from the same IP, the triple is recognised, and the invoice is accepted and delivered. The client sees the message with only a few minutes of delay.
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Frequently Asked Questions
No, it defers it. Legitimate email is delivered after a short retry delay. Only senders that do not retry properly lose mail to greylisting.
The retry delay is often around five minutes, though some servers require longer. The sender is then remembered for a period that can range from hours to a month, during which further mail is accepted without deferral.
Delays usually occur because the sending server retries too slowly, retries from a different IP, or does not retry at all. Reliable, consistent retry behaviour from a fixed IP avoids most greylisting delays.
No. Greylisting defers the first message from an unknown sender to test retry behaviour, while email throttling limits the rate at which a sender's mail is accepted. Both can delay delivery, but they operate on different principles.