535 5.7.57 SMTP; Client was not authenticated to send anonymous mail
535 5.7.57 SMTP; Client was not authenticated to send anonymous mail
A different problem from the ones above: the client sent no credentials at all, or sent them after MAIL FROM instead of before. Usually a device or script configured for anonymous internal relay that is now pointed at an endpoint requiring authentication. Check whether the SMTP settings have an authentication checkbox that is switched off, before assuming Basic auth is the issue.
Check it from the machine that is failing
npx zerosmtp-check --explain "535 5.7.57 SMTP; Client was not authenticated to send anonymous mail"
No install and nothing sent. It reads the refusal your own client printed - which is rarely what the server said, because libraries and device panels rewrite it - and says which of these cases you are in.
If the send is hanging rather than being refused, the cause is usually the network and not the credentials. npx zerosmtp-check with no arguments checks ports 25, 587 and 465 from where you are standing.
Check this before assuming it is the Basic auth shutdown
This one is worth ruling out cheaply, because the fix is usually local:
- Does the device’s SMTP configuration have an authentication checkbox, and is it off?
- Was this device previously relaying anonymously through an internal server that has since been retired or repointed?
- Is it sending
AUTHafterMAIL FROM? Some older firmware does, and Microsoft 365 will not accept it.
If authentication is genuinely configured and still refused, you are looking at 535 5.7.139 instead.
Two things this is often confused with
| Symptom | Actual cause |
|---|---|
| Connection times out; no authentication error ever appears | The network is blocking outbound SMTP. Cloud providers block port 25 and often 587 by default — see troubleshooting. |
Certificate verify failed / unable to get local issuer certificate | The device’s trust store cannot validate the server certificate. Common on firmware predating current root CAs — see the Canon Maxify MB2755 case. |
Related
- All SMTP AUTH error messages
- Which devices have OAuth firmware
- What breaks at the end of December 2026
Seeing a string that is not here? Report it with what produced it. Errors from real hardware are worth more than anything transcribed from documentation.
Last reviewed 2026-08-16.
Updated 22 Aug 2026