SMTP error 5.7.24 SPF validation error

What SMTP error 5.7.24 means

This is not SPF saying no. It is SPF unable to produce an answer at all: the receiver tried to evaluate your record and the evaluation itself broke. It appears as 451 4.7.24 where the receiver believes the problem may clear, and 550 5.7.24 where it has decided the record is permanently unusable. The fault is nearly always in your own record — a syntax error, a duplicate, or more DNS lookups than the standard permits.

Security or policy status
Sent by The receiving mail server, or a gateway carrying out the SPF evaluation on its behalf
Occasional

What SMTP error 5.7.24 SPF validation error actually means

RFC 7208 defines two error outcomes that are separate from a fail. A temperror means DNS did not answer and the check could not be completed; a permerror means the record was retrieved and could not be processed. Receivers signal the first with 451 and the second with 550, and this code is the only one in the authentication group registered for both classes, so the reply number in front of it carries real information. That is also the whole distinction from 5.7.23: a fail says the connecting address is not authorised, whereas an error says nobody can determine whether it is authorised or not. A 451 will be retried by the sending server for a day or more and often recovers by itself. A 550 never will.

This status code is returned when evaluation of SPF relative to an arriving message resulted in an error. Used in place of 4.4.3 or 5.5.2 as described in Sections 8.6 and 8.7 of [RFC7208].
— RFC 7372 with RFC 7208

How 5.7.24 appears in a bounce

A server reporting this condition sends it alongside a three-digit reply code, and a bounce prints the two together. The registry lists 550 5.7.24 as the pairing for this status. The registry also lists 451 for this condition, but that belongs with 4.7.24 rather than with this code, because a reply code and the status beside it always agree on whether the failure is temporary or permanent.

The leading 5 is the server's verdict rather than part of the code's identity: it marks this as a permanent failure, which means the sending server has given up and returned the message. The same condition reported as 4.7.24 is the other verdict on the identical problem, so a log showing that form has not produced a bounce yet.

What causes SMTP error 5.7.24 on a business phone system

More than ten lookup-consuming mechanismsThe include, a, mx, ptr, exists and redirect mechanisms all count, and includes nest, so four visible includes can resolve to a dozen queries. Crossing the limit is a permanent error whatever the connecting address happens to be.
Two records starting v=spf1 on the same nameThe standard makes this a permanent error in itself, with no attempt to merge or pick one. It appears when a provider setup wizard publishes its own record beside the existing one instead of asking you to combine them.
A hand-edited syntax mistakeA comma between mechanisms, an https:// prefix pasted into an include, a missing v=spf1 at the front, or a control panel adding quotation marks around a value you already quoted. Any of these makes the record unparseable.
Too many void lookupsLookups that return nothing at all are separately capped at two. An include pointing at a service you cancelled last year is a void lookup, and a couple of those tip the evaluation into an error even though the rest of the record is sound.
Your authoritative DNS not answeringNameservers under load, a broken DNSSEC signature or an expired zone produce temperror at every receiver at once. If failures started at one minute past the hour and affect everyone, look at DNS rather than record content.

How to fix SMTP error 5.7.24

Note whether the reply was 451 or 550 before anything elseThat single digit separates a DNS availability problem from a broken record, and the two have nothing in common. Chasing record syntax when the real issue was a nameserver outage wastes an afternoon.
Resolve the record fully and countUse a validator that expands nested includes and reports the total, because counting the includes you can see is not the same measurement. Anything at nine or ten needs reducing now, not when it breaks.
Remove includes for services you no longer useGo through the record line by line and ask who still uses each entry. Cancelled platforms leave both dead weight and void lookups behind, and pruning them is usually enough to bring the count back into range on its own.
Prove there is exactly one recordQuery the TXT records for the bare domain and count how many begin v=spf1. If there are two, delete one rather than editing both, and check any subdomains you send from while you are there.
Check how your DNS host stores long valuesA single TXT string cannot exceed 255 characters, and control panels differ in how they split longer values. Read the record back as it is actually served rather than as it appears in the editing form.

Fixing the underlying problem

This page explains the code. These guides walk through the fix in detail.

Questions about SMTP error 5.7.24

The bounce shows 451. Will the message get there eventually?

If the cause was transient, yes. Sending servers queue on a 451 and retry over the following day or so before giving up and issuing a final non-delivery report. If the same messages later come back as 550, the receiver has concluded the problem is not going to clear.

We only have six includes. Why are we over the limit?

Because includes contain includes. A single entry for a large platform can resolve to three or four queries of its own, and the standard counts the total resolved, not the number of lines in your record. Use a tool that expands the whole tree.

Would changing -all to ~all get us delivering again?

No. The all qualifier only decides what happens to addresses the record does not cover, and an error means the record was never successfully processed to that point. Fix the record itself.

Related SMTP status codes

Source. SMTP error 5.7.24 SPF validation error is defined in RFC 7372 with RFC 7208 and registered in the IANA SMTP Enhanced Status Codes registry. The causes and fixes above are drawn from our own experience supporting UK business email systems.

All Email delivery & SMTP codes