What greylisting is
Greylisting is a technique receiving servers use to filter spam without inspecting content. When
an unfamiliar sender tries to deliver mail, the server temporarily refuses with a 4xx code — usually
451 or 450 — and asks the sender to retry later.
Legitimate mail servers follow the SMTP retry rules and try again after a short delay. Most spam senders never retry, so the message never lands. That's the entire trick.
How to spot greylisting in your logs
- The response is a 4xx code, not 5xx. It's temporary.
- The reason string often says "Greylisted", "try again later", or "temporary local problem".
- The retry succeeds — usually within 5–30 minutes.
- Subsequent sends from the same IP/domain to the same receiver deliver immediately (most greylisters cache the sender).
This is very different from a permanent block, which returns a 5xx. If you're seeing 5xx, the 550/554 SMTP error guide is the right reference.
What to do when you're greylisted
- Nothing urgent. Retry policy handles it automatically for reputable ESPs.
- Don't suppress the address. A 4xx is not a bounce.
- Confirm your MTA retries with backoff. The standard retry window is roughly 15 minutes, then 30, then 1 hour, etc.
- Warm the sender/receiver pair. Once you've delivered once, the receiver typically stops greylisting your IP/domain for that recipient.
How greylisting relates to your overall bounce picture
Greylisting shows up as a soft failure. It should not push your hard bounce rate. But if you count every deferral as a bounce in your reporting, you'll overstate the problem. Compare the two:
- Read soft bounce vs hard bounce so your suppression rules don't remove valid addresses.
- Sanity-check your overall numbers against email bounce rate benchmarks.
- Focus long-term wins on improving inbox placement rate, which reduces both content-based deferrals and greylisting friction over time.
When greylisting becomes a real problem
If you're sending time-sensitive transactional mail (password resets, magic links, order confirmations), even a 15-minute delay is a bad user experience. In that case:
- Use a dedicated transactional ESP with a strong sending reputation — many receivers whitelist those IPs.
- Warm up the sending domain so receivers stop treating you as unfamiliar (see the domain warm-up guide linked below).
- Monitor deferral rates in your ESP dashboard, not just bounce rates.
Key takeaways
- Greylisting is a 4xx temporary deferral, not a bounce
- Legitimate senders retry automatically and deliver on the second attempt
- Never suppress an address just because it was greylisted once
- Persistent greylisting on a new sending domain usually means you need to warm the domain
- Confirm your list is clean of the addresses that actually cause hard bounces with the free ListEmailCheck bulk verifier.