/ Glossary — Reputation & Bounces
What is greylisting?
An anti-spam tactic that temporarily rejects the first delivery attempt from an unknown sender, betting that real servers retry and spam tools don't. Here's how it works.
/ Definition
Greylisting is an anti-spam technique in which a receiving mail server temporarily rejects a message from an unfamiliar sender — returning a temporary failure rather than accepting it — and waits to see whether the sender tries again. Legitimate mail servers follow the rules and retry after a short delay, at which point the mail is accepted; many spam tools, built to fire and forget, never retry.
The effect on legitimate senders is a one-time delay: the first message to a given receiver is deferred and arrives minutes later on retry, after which that sender is usually whitelisted. It registers as a temporary soft bounce followed by a successful delivery, which is normal and harmless as long as your sending infrastructure retries correctly.
/ In practice
How greylisting works
When a greylisting server receives mail from a sender it hasn't seen before — identified by a combination of sending IP, envelope sender, and recipient — it responds with a temporary failure code, the SMTP 4xx range, telling the sender to try again later. A legitimate server queues the message and retries after its normal delay, and on that second attempt the greylisting server accepts it, often adding the sender to a known-good list so future mail passes without delay. The technique exploits a simple asymmetry: real mail servers are built to retry on temporary failures, while a lot of bulk spam software simply moves on.
What it means for senders
For a properly configured sender, greylisting is mostly invisible — a brief, one-time delay on first contact, then normal delivery. The only thing that turns it into a problem is a sending setup that doesn't retry correctly on a soft bounce, in which case greylisted mail never gets its second attempt and silently fails to deliver. This is one more reason a competent MTA matters: handling temporary failures with correct retry behavior is exactly what lets your mail sail through greylisting the way legitimate mail is supposed to.
/ Greylisting today
A technique in decline, but not gone
Greylisting was more widespread a decade ago than it is now, and its use has declined as spam filtering grew more sophisticated and as the delay it imposes became less acceptable in an era of instant expectations. Many receivers have moved to reputation-based filtering that doesn't need the deliberate delay greylisting introduces. But it hasn't disappeared — plenty of servers, especially smaller or self-hosted ones, still use it, so a sender mailing a broad audience will still encounter it.
The trade-off it represents
Greylisting's decline reflects a real trade-off. It's cheap and catches unsophisticated spam tools that never retry, but it delays legitimate first-contact mail by minutes, which matters for time-sensitive messages like password resets or one-time codes — exactly the mail you least want delayed. A receiver running greylisting accepts that cost in exchange for the spam it blocks. For senders, the lesson is unchanged regardless of how common greylisting remains: a sending setup that handles temporary failures with correct retry behavior passes through greylisting transparently, while one that doesn't will see mail mysteriously fail to reach the servers that use it.
/ Related terms
Greylisting is harmless if you retry right. Our infrastructure does.
Correct retry handling on temporary failures is built into how we operate sending, so greylisting stays invisible. Bring us your sending and we'll handle the deferrals.
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