451 4.7.1 "Try Again Later": Greylisting, Policy Deferrals and When to Worry

451 4.7.1 means the receiving server deferred your message for now, most often because of greylisting or a temporary policy check. It is a soft bounce. A correctly set up sending server retries after a few minutes and the message usually goes through. It only becomes a problem if the retries keep failing.

The 451 4.7.1 code page has the short version. For the 450 flavour, see greylisted 450 try again later.

Reading the code

Part	Value	Meaning
Reply code	451	Requested action aborted, local error
Class	4	Persistent transient failure
Subject	7	Security or policy status
Detail	1	Delivery not authorized, message refused

The class and subject come from RFC 3463. Servers pick the wording themselves, which is why the same code can mean different things.

What causes it

• The receiving server greylists mail from senders it has not seen before. It rejects the first attempt and accepts a retry.
• A temporary policy or reputation check deferred the message.
• The sending IP changed between attempts, so greylisting started again.

What to do

1. Make sure your tool retries deferred mail. Tools that treat any 4xx as final will lose greylisted messages. SMTP deferral vs bounce explains the difference.
2. Read the text if it keeps failing for hours. Some servers use 451 4.7.1 for blocklist or reputation checks, which need a different fix.
3. Send from a consistent IP. Greylisting tracks the sending IP, so rotating IPs restarts the delay.
4. Do not suppress the address on a single deferral. Soft bounce handling covers when to give up.

Greylisting and list verification

A greylisting server gives the same temporary answer to a mailbox check as it does to a real message. That is why a verifier marks such an address unconfirmed rather than valid or invalid: it cannot tell yet whether the mailbox exists. Why SMTP verification gives false positives covers where those limits sit.

Is it ever a real block

Yes. If the same 451 4.7.1 repeats across many retries and many days, treat it like a block rather than greylisting. Compare the wording with Gmail 421 4.7.0 and Microsoft 451 4.7.650.

Keep the address side clean

This code does not come from your list, but every bounce still counts against your sending reputation, so the addresses you do send to should be real. Upload your list on the verify page: the first 100 email checks are free with a Google sign-in, no card. Each address comes back valid, risky or invalid with a reason such as "Mailbox not found" or "Accepts all addresses", and nothing is sent to anyone on the list. For larger or recurring lists, see bulk email verification, list cleaning and pricing.