What are graylist-related deferrals?
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Greylisting is an anti-spam technique where a mail server temporarily rejects an email the first time it sees it from an unfamiliar sender and asks the sending server to try again. The idea is based on a simple observation: spam bots usually don't retry. Legitimate mail servers do.
When a greylist deferral happens, you'll see a 4xx response code (usually 451) in your bounce log, with a message like "451 Try again later" or "451 Greylisted, please try again." This is not a failure. It's a temporary hold.
The sequence looks like this:
- Your server connects to the receiving server and attempts delivery.
- The receiving server returns a 451 and records your IP address, the sending domain, and the recipient address in its greylist.
- Your server waits the required retry period (usually 5-30 minutes) and tries again.
- On the retry, the receiving server recognizes you as a legitimate sender and accepts the message.
For most senders using a reputable ESP, this happens invisibly. Your ESP's mail transfer agent handles the retry automatically, and the only trace is a slightly longer delay before delivery. Most recipients never notice.
Where it matters: if you're sending from your own mail server or a custom SMTP setup, make sure it's configured to retry on 4xx responses. Some older or misconfigured setups bounce 4xx codes as permanent failures, which would cause legitimate greylist deferrals to show up as bounces. Check your soft bounce settings if you're seeing a high rate of 451 codes that aren't resolving.
Greylist deferrals are essentially a sign that the receiving server doesn't know you yet, not that anything is wrong. Once your sending IP and domain have a history with that server, greylist delays typically shorten or disappear.
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