SMTP error 454 4.7.0: temporary authentication failure

SMTP error 454 4.7.0 is a temporary authentication failure. The 4 leading digit is the whole

story: this is not a rejected credential, it is the server's authentication backend being briefly

unable to process the AUTH command at all - an LDAP or directory lookup timing out, a rate limit

on failed-login checks, or the auth service restarting.

Fault: usually the receiving server, sometimes a sender-side retry storm. Treat it as a

deferral, not a reason to change your password or regenerate an app password.

Why it happens

• The authentication backend is unavailable. Many large mail systems check credentials

against a separate directory service (LDAP, a token-validation API). If that dependency is

slow or down, the SMTP layer reports 454 4.7.0 rather than a hard 535.

• Too many failed authentication attempts from one IP in a short window. Some providers

temporarily throttle *all* auth attempts from a source, including correct ones, after a burst of

failures - often triggered by a different, misconfigured sender sharing your IP or NAT gateway.

• The receiving server is mid-restart or mid-deployment of its auth component.
• **A load balancer routed your connection to a node that has not yet synced recent credential

changes** - common right after a password or app-password rotation.

• The receiving provider is absorbing an unrelated incident elsewhere in its stack - a

database failover, a certificate rotation, or a capacity event in a completely different

component can still ripple into the authentication path, since most large providers share

infrastructure between services that look unrelated from the outside.

This last cause is worth remembering specifically because it means the fix is patience, not

configuration. There is no setting on your side that compensates for a receiving provider's own

internal incident, and trying to find one wastes time that is better spent simply waiting out the

retry window.

The transcript

$ swaks --to test@mailpilots.in --server smtp.example.net:587 \
        --auth LOGIN --auth-user sender@yourdomain.com -tls
-> AUTH LOGIN
<- 334 VXNlcm5hbWU6
-> c2VuZGVyQHlvdXJkb21haW4uY29t
<- 334 UGFzc3dvcmQ6
-> ********
<- 454 4.7.0 Temporary authentication failure

Run the exact same command thirty seconds later with the exact same credential - if it succeeds,

you have confirmed it was transient and there was nothing wrong with the login.

How the big three word it

Diagnose it

$ for i in 1 2 3; do
    swaks --to test@mailpilots.in --server smtp.example.net:587 \
          --auth LOGIN --auth-user sender@yourdomain.com --auth-password "$PASS" -tls \
          2>&1 | grep -E "^<- (454|535|250)"
    sleep 30
  done
<- 454 4.7.0 Temporary authentication failure
<- 454 4.7.0 Temporary authentication failure
<- 235 2.7.0 Authentication successful

Three attempts thirty seconds apart, succeeding on the third, is the signature of a transient

backend issue rather than a wrong credential - a wrong credential fails identically every time.

The fix, in order

1. Wait sixty seconds and retry the identical credential. Do not change anything about the

login on the first retry.

1. If it still fails after several minutes, check the provider's status page - this error

correlates strongly with brief, provider-side auth-service incidents.

1. If you are self-hosting, check your SASL/LDAP backend's own logs (doveconf, slapd

logs) for timeouts independent of the SMTP log.

1. Back off your retry rate. Hammering AUTH every few seconds during an outage can get your

IP additionally throttled on top of the original cause.

1. Only after repeated, spaced-out failures should you suspect the credential itself and move

to checking whether a password or app-password was rotated elsewhere.

What not to do

• Do not regenerate an app password on the first failure. If the credential was fine, you

have just created a new reason to fail (propagation delay for the new credential).

• Do not retry in a tight loop. Rapid retries are themselves what triggers some providers'

auth throttling, turning a one-off hiccup into a sustained block.

• Do not confuse this with `535 5.7.8`. That code is permanent and about the credential

itself; 454 4.7.0 is temporary and about the service checking it.

• Do not disable TLS "to rule it out". It will not affect an authentication-backend outage

and only weakens the connection's security.

MailPilot connects Gmail and Outlook mailboxes over OAuth, which removes this failure mode for

those providers entirely - there is no password-based AUTH command to be deferred. For

SMTP/IMAP-connected providers it retries failed connection attempts on a schedule rather than

immediately, which is exactly the right response to a 454 4.7.0.