Unable to send email via smtp

I have the same smtp information registered for all development, staging, and production, but I get errors sending emails outside of the development environment.

I would appreciate it if someone could tell me what the cause is.

Log of sending error]
{
“date”: “2025-06-26T12:16:24.739Z”,
“type”: “fn”,
“description”: “To: {address}”,
“connection_id”: “”,
“ip”: “221.103.226.216”,
" details":. {
“email_type”: “try_provider_configuration_email”,
“notification_type”:. “try_provider_configuration_email”,
“to”: “{address}”,
“error”:. “Error sending email:. Invalid login: 535 Error: authentication failed, authentication failed, authentication failed, authentication failed, authentication failed 535 Error: authentication failed, system busy”
},
“user_id”:“”, “user_name”. “”,
“user_name”: “{address}”,
“environment_name”: “prod-jp-1”,
“log_id”:. “900202506261216838394000000000000001223372040953481646”,
“tenant_name”: “{tenant}”,
“_id”:. “90020250626121624838394000000000000001223372040953481646”,
“isMobile”: false,
“user_agent”: “false”. “Other 0.0.0 / Other 0.0.0”,
“id”: “90020250626121624838394000000000000001223372040953481646”
}

Translated with DeepL.com (free version)

Hi @Koki-1205,

Welcome back to the Auth0 Community!

The error above is an authentication error happening on the Email Provider’s end and is likely caused because the SMTP provider rejects the request due to the fact that the submitted credentials do not have the necessary permissions.

You can check out this knowledge article that provides the steps needed to resolve this issue.

I hope this helps!
Thanks,
Remus

Thanks to @remus.ivan
for contacting me.

I checked the article, but I am using a custom smtp provider and it didn’t seem to be helpful.
(I use Lark provided by ByteDance to manage my email).

Also, what I am most curious about is why only smtp in the development environment is working correctly.

I am using the same information in all environments, do you know why it is not working outside of the development environment?

Hi @Koki-1205,

The error message described above is pointing out exactly where the issue resides, being an authentication error on the custom SMTP provider’s end. The reason why this might still be working in the development environment is for testing purposes, presuming that the IP address is implicitly trusted by Lark’s SMTP server or within a permitted range.

But when your staging/production servers try to connect, their IPs are not recognized, leading to authentication failures, so the reason why this is not allowed on other environments is because of firewall restrictions, so the associated IP addresses would have to be whitelisted on the provider’s end. This is also mentioned in our Auth0 IP Addresses for Allow Lists documentation

if you’ve configured an on-premise SMTP provider in Auth0, then you may need to configure your firewall to allow inbound traffic from Auth0.

but I would also advise having a look at our Email provider setup information.

Hopefully this helps a bit more!
Kind regards,
Remus

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