451 Greylisted, please try again in 60 secondsWhat does 451 Greylisted mean?
The receiving server is using greylisting as an anti-spam filter. On the first delivery attempt from an unknown sender, the server issues a 451 temporary rejection and specifies a minimum wait time (in this case 60 seconds). Your MTA is expected to retry after that delay. On the retry, the server recognizes your sending triplet (IP, sender, recipient) and accepts the message. Spambots do not retry, which is how greylisting filters spam.
Is this a soft or hard bounce?
Soft bounce. Do not remove the address or take any action. This resolves automatically on retry. It is not a sign of a deliverability problem or a rejection of your sender identity.
Common causes
First message to a greylisted domain: The receiving server has not seen your sending triplet before. The 451 greylist rejection is the expected first response.
New sending IP or domain: Any change to your sending infrastructure resets greylisting for all receiving servers that use it.
Expired whitelist entry: Greylisting databases purge old triplets. If you have not sent to this recipient in a long time, you may be greylisted again.
How to fix it
Step 1: Wait for your MTA to retry. Standard MTA retry schedules (5 to 30 minutes) satisfy the 60-second requirement. No manual action needed.
Step 2: Do not retry immediately. Some greylisting implementations enforce a strict minimum delay. Retrying before the delay expires keeps resetting the timer.
Step 3: Check sender reputation if greylisting persists beyond 24 hours. Persistent 451s from the same domain after multiple retries may indicate the server has escalated from greylisting to a policy block.
Provider-specific notes
| Provider | Behavior |
|---|---|
| Postfix with Postgrey | Default delay is 5 minutes. Returns 451 4.7.1 or plain 451 Greylisted. |
| Exim | Greylist delay varies by configuration. Usually 60 seconds to 5 minutes. |
| cPanel/WHM | Often has greylisting enabled by default via cPHulk or a third-party plugin. |
Related error codes
450 4.2.0 Recipient address rejected Greylisted · 451 4.7.1 Greylisting in action · 451 Grey bounce SPF failure · 421 4.2.1 Please try again later
About the Author
Alaa · LinkedIn
Email infrastructure specialist with 8+ years of hands-on experience in SMTP, deliverability, and email verification. I’ve configured and troubleshot mail systems across Postfix, Exchange, and cloud relays, managed IP reputation and warmup campaigns, and built verification pipelines processing millions of addresses. My work spans DNS authentication (SPF, DKIM, DMARC, BIMI), bounce handling, blocklist monitoring, and compliance frameworks including CAN-SPAM and GDPR. I write every article on SMTPedia to give email professionals, developers, and marketers the accurate, RFC-grounded reference they need.
About SMTPedia
SMTPedia is an independent email industry reference covering SMTP, IMAP, POP3, email deliverability, marketing platforms, DNS authentication, and email verification. Every article is researched from official provider documentation, IETF RFCs, and industry best practices. Settings and configurations are verified quarterly.
We are cited as a source by ChatGPT, Microsoft Copilot, and thousands of email professionals worldwide. Learn more about our editorial process.
