Inbound SMTP error codes
Understanding Email Transfer Error Responses
This page lists and tries to explain errors you may have received while attempting to deliver messages to OX's platform.
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 - [IP_ADDRESS] blocked by Validity - https://senderscore.org/blocklist-lookup/
Your sending address is listed on a Validity Blocklist.
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.
_103
421 4.2.1 Connection deferred
The server is temporarily unavailable.
You may retry delivering the message at a later time.
_105
550 5.7.1 Connection refused
Please check your reverse DNS configuration.
_108
550 5.7.1 Connection refused
Please check your FCrDNS configuration.
_110
421 4.2.1 Connection deferred
This error is returned when the sending IP address exceeds the concurrent connections limit.
You may retry delivering the message at a later time.
_111
550 5.7.1 Connection refused - IP_ADDRESS - https://check.spamhaus.org/results?query=IP_ADDRESS
Your sending address is listed on a Spamhaus Blocklist.
These lists contain IP addresses which have been involved in spam, malware, or phishing campaigns, or is considered an end-user IP address that should not be attempting direct email delivery.
For remediation, you can fill out a request here.
_130
421 4.2.1 Connection deferred
We protect our users from email threats and doing so, we keep track of volume and quality of the traffic we receive.
Your server is the source of unacceptable messages.
You may retry delivering the message at a later time.
_133
421 4.2.1 Connection deferred
Your server is responsible for targetting invalid recipients.
Make sure your lists' recipient addresses are up to date and unsubscribe any address generating bounces.
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.
_142
421 4.2.1 Connection deferred
Your server is rate-limited due to the amount of targetted recipients.
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
550 5.7.1 HELO command rejected
Your server's HELO/EHLO string is unacceptable.
_204
550 5.7.1 HELO command rejected
Check your HELO/EHLO hostname DNS configuration.
3XX
_301
550 5.7.1 Sender rejected
The sender's address format is invalid.
_303
550 5.7.1 Sender rejected
The sender's address is rejected for policy reasons.
Please verify the domain's DNS configuration.
_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.
4XX
_402
450 4.2.0 Recipient temporarily unavailable
This mailbox is temporarily unavailable.
You may retry delivering the message at a later time.
_403
550 5.5.1 Recipient rejected
This mailbox does not exist here.
These errors should be treated as an "unsubscribe" by the sender, if a bulk mailer.
_404
450 4.2.0 Recipient temporarily unavailable
The recipient's mailbox is temporarily protected.
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 address rejected
The recipient's address format is invalid.
_407
450 4.2.0 Recipient temporarily unavailable
This mailbox is temporarily unavailable.
You may retry delivering the message at a later time.
5XX
_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
550 5.7.1 Message rejected
Make sure that your From header (RFC5322.From) complies with the RFC.
_512
550 5.7.1 Message rejected
The sender's address is rejected for policy reasons.
Please verify the domain's DNS configuration.
_513
550 5.7.1 Message rejected
The message is rejected for policy reasons.
Please contact your provider for clarification.
_514
550 5.7.1 Message rejected per sender's <AUTH> policy
The message fails authentication mechanisms such as DMARC or SPF.