SMTP error 451 4.7.23: SPF validation failure

451 4.7.23 pairs a temporary 4xx with 4.7.23, an enhanced status code some providers use

for SPF validation failure specifically — the 4.7.x range covers security and policy conditions

under RFC 3463. The 4 is the important part: the receiver has not permanently rejected the

message, it is deferring while SPF fails to validate, giving the sending side a retry window to fix

the underlying record.

A real transcript

MAIL FROM:<you@yourdomain.com>
250 2.1.0 Sender OK
RCPT TO:<recipient@example.com>
250 2.1.5 Recipient OK
DATA
354 Start mail input
...
451 4.7.23 SPF validation failed, try again later

How the big three word SPF failures

Why SPF authentication matters here

SPF lets a receiving server check whether the IP that is actually sending the mail is authorised,

per DNS, to send on behalf of the envelope sender's domain. When that check fails, some receivers

defer rather than hard-reject, which is what produces this specific code instead of an outright

550.

Common causes

1. Missing or misconfigured SPF record — no TXT record at all, or a record with a typo in a

mechanism (include:, a, mx, ip4).

1. Exceeding the SPF DNS lookup limit — RFC 7208 caps SPF evaluation at 10 DNS lookups; past

that, the result is permerror, which many receivers treat as a failure.

1. Email forwarding — a forwarder resends your mail from its own IP, which is not in your SPF

record, so SPF fails for the forwarded copy even though your original send was fine.

1. Unauthorised third-party senders — a marketing tool or CRM sending on your domain's behalf

without being added to your SPF record.

1. Conflict with DKIM/DMARC configuration — if DMARC alignment is also broken, some receivers

escalate what would otherwise be a minor SPF soft-fail.

Diagnose it yourself

1. Pull your SPF record directly: dig TXT yourdomain.com +short — confirm exactly one record

starts v=spf1 and ends in -all or ~all. Two SPF TXT records at the same name is itself an

error that makes SPF permerror regardless of either record's content.

1. Count the lookups by hand: list every include:, a, mx, exists, and redirect

mechanism in the record and in everything it includes, recursively — more than 10 total is a

permerror.

1. Confirm the connecting IP matches what SPF authorises: dig A yourdomain.com +short for an

a mechanism, or check the literal ip4:/ip6: ranges against the IP your sending platform

actually uses — a platform that rotates sending IPs without updating your SPF record is a common

silent cause.

1. Read the `Received-SPF` or `Authentication-Results` header on a test message sent to a

mailbox you control — it states the SPF verdict and the exact IP and domain it checked.

How to fix SMTP Error 451 4.7.23 and SPF failures

1. Verify your SPF record exists and is well-formed — one TXT record starting v=spf1, ending

in an all mechanism (-all or ~all).

1. Reduce DNS lookups in SPF. Each include:, a, mx, exists, and redirect costs a

lookup; flatten nested includes or remove unused ones to stay under 10.

1. Address forwarding-related SPF failures separately — SPF cannot be fixed for forwarded mail

by changing your own record; DKIM (which survives forwarding) is the mechanism that matters

there.

1. Authorise third-party email services — add every legitimate third-party sender to your SPF

record via its documented include: mechanism, checking every platform you actually send from,

not just your primary one.

1. Align SPF with DKIM and DMARC for stronger authentication. Even a technically passing SPF

record does nothing for DMARC if it is not aligned with the From: header domain — fix DKIM

alignment as the more durable mechanism.

What not to do

• Do not treat this as a bounce — it is a 4xx deferral, and the message is still queued; marking

the recipient as undeliverable throws away someone who will likely receive the retried message

fine.

• Do not add +all to your SPF record to make the error stop. That authorises any server on the

internet to send as you, which several receivers treat as a spam signal in itself and can make

your overall deliverability worse, not better.

• Do not stack more include: statements without checking your lookup count — this can push you

past the 10-lookup limit and turn a soft SPF failure into a hard permerror, which several

receivers treat even less favourably.

• Do not ignore forwarded-mail failures as unfixable — DKIM alignment is the actual fix for that

case, not an SPF change, and giving up on it leaves a real deliverability gap for anyone who

forwards your mail.

MailPilot checks SPF, DKIM, and DMARC for every connected mailbox and reports lookup-count and

alignment problems directly, so a 451 4.7.23 traces back to a specific broken mechanism in your

DNS rather than requiring you to read raw records by hand.