Outbound SMTP error codes

Understanding Email Transfer Error Responses

This page lists and tries to explain errors you may have received while attempting to send messages using OX's service.
The SMTP response you received contains a unique code starting with OXSUS_.
Find the code below to understand the issue you are experiencing and find remediation tips.

Info

All policies and thresholds listed here are subject to change.

1XX

_101

550 5.7.1 Connection refused - [IP_ADDRESS] blocked by Vade Threat List

Your sending address is listed on the Vade Threat Network List.
This list contains IP addresses which have been involved in spam, malware and/or phishing campains.

For remediation, you can fill out a request here.

_102

550 5.7.1 Connection refused

Your sending address is listed in our blocklist.
This list contains IP addresses which have been involved in spam, malware and/or phishing campains.

_130

421 4.2.1 Connection deferred

Your server is the source of unacceptable messages.
In order to preserve our deliverability, your IP address is temporarily rate-limited.
You may retry delivering the message at a later time.

_134

421 4.2.1 Connection deferred

Your server was detected to take part of SMTP AUTH attacks.
In order to preserve our deliverability, your IP address is temporarily rate-limited.
You may retry delivering the message at a later time.

_136

421 4.2.1 Connection deferred

Your server is rate-limited due to excessive volume sent.
You may retry delivering the message at a later time.

_139

421 4.2.1 Connection deferred

Your server is rate-limited due to excessive connections received.
You may retry delivering the message at a later time.

2XX

_201

550 5.7.1 HELO command rejected

Your server's HELO/EHLO string is unacceptable.

_202

535 5.7.8 Authentication refused

Your sending account is listed on our blocklist.
Please contact your provider for clarification.

_206

421 4.2.1 Connection deferred

Your sending account has been involved in spam, malware and/or phishing campains and is therefore temporarily suspended.
You may try again later.

_207

550 5.7.1 Authentication Refused

Please check your AUTH username and password.

_209

435 4.7.13 Authentication refused

Your sending account is rate-limited due to its login history.
You may try again later.

_210

435 4.7.13 Authentication refused

Your sending account is rate-limited due to its login history.
You may try again later.

3XX

_301

550 5.7.1 Sender rejected

The sender's address format is invalid.

_305

450 4.2.1 Sender temporarily rejected

This error is returned when the sending IP address exceeds the messages per session limit.
You may retry delivering the message at a later time.

_306

550 5.7.1 Sender rejected

The sender's address is rejected for policy reasons.
Please contact your provider for clarification.

_307

421 4.2.1 Sender temporarily rejected

Your account is the source of unacceptable messages.
In order to preserve our deliverability, your account is temporarily rate-limited.
You may retry delivering the message at a later time.

_309

550 5.7.1 User not allowed to send

Your account is disabled, please contact your provider for clarification.

4XX

_402

550 5.7.1 Recipient rejected

The recipient's address is rejected for policy reasons.

_404

450 4.2.0 Recipient temporarily rejected

Your server is rate-limited due to excessive volume sent.
You may retry delivering the message at a later time.

_405

450 4.2.0 Recipient temporarily rejected

Your message reached the authorized number of recipients per message limit.
Impacted recipients should be accepted on next retries.

Try reducing the number of recipients per message.

_406

550 5.7.1 Recipient rejected

The recipient's address format is invalid.

_407

550 5.7.1 Recipient rejected

The recipient's domain is invalid.

5XX

_502

550 5.7.1 Message rejected

The messsage is rejected for policy reasons.

_503

550 5.7.1 Message rejected

Please check your RFC5322.From value.

_507

451 4.3.0 Message temporarily rejected

The server is temporarily unavailable.
You may retry delivering the message at a later time.

_510

552 5.3.4 Message size limit exceeded

_511

451 4.3.0 Message temporarily rejected

Your account is rate-limited due to excessive volume sent.
You may retry delivering the message at a later time.

_513

550 5.7.1 Message rejected

The message is rejected for policy reasons.
Please contact your provider for clarification.

_514

554 5.6.0 Too many hops

The message is suspected to be looping between servers.

_539

451 4.3.0 Message temporarily rejected

Your server is rate-limited due to excessive volume sent.
You may retry delivering the message at a later time.