Understanding Email transfer error responses

This page lists and tries to explain errors you may have received while attempting to deliver messages to our servers. Some errors also provide tips for you to remediate the block. The SMTP response you received contains a unique code starting with OXSUS_; Find the code below to spot the block reason you are experiencing.

OXSUS00X_101
550 5.7.1 Connection refused
We could not accept your email because your email provider is listed on the Vade Secure Threat Network list.

This list contains IP addresses which have been identified as sending spam. We use this list to protect our subscribers from receiving spam.

OXSUS00X_102
550 5.7.1 Connection refused
We could not accept your email because your email provider is listed on the Validity Blocklist. Please go to https://senderscore.org/blocklist-lookup/ to get unlisted

This list contains IP addresses which have been identified as sending spam. We use this list to protect our subscribers from receiving spam.

OXSUS00X_103
421 4.2.1 Connection deferred
We could not accept your email at that moment because the server is experiencing issues delivering to backend servers. There are too many messages in queue at the moment. You may retry delivering the message at a later time.
OXSUS00X_105
550 5.7.1 Connection refused
Reject if reverse dns is not present
OXSUS00X_110
421 4.2.1 Connection refused
We could not accept your email because the email provider has opened too many connections from a single IP address. The message may be retried using a different SMTP session.
OXSUS00X_130
421 4.2.1 Connection refused
Our servers protect our users by checking the spam ratio that was received from your mail delivery system over a given time period. This message informs you that your mail delivery system has exceeded the spam ratio over a given period of time, and that the message may be retried at a later time.
OXSUS00X_133
421 4.2.1 Connection refused
Our servers protect our users by checking the number of invalid recipients over a given time period. This message informs you that your mail delivery system has reached the maximum number of bad recipients over a given period of time, and that the message may be retried at a later time.
OXSUS00X_136
421 4.2.1 Connection refused
Our servers rate limit all inbound email traffic, in order to protect our customers and our email infrastructure from attacks. This message informs you that the IP address of the mail delivery system has reached its limit by sending too many messages, and that the message may be retried at a later time.
OXSUS00X_139
421 4.2.1 Connection refused
Our servers rate limit all inbound email traffic, in order to protect our customers and our email infrastructure from attacks. This message informs you that the IP address of the mail delivery system has reached its limit by opening too many connections, and that the message may be retried at a later time.
OXSUS00X_140
421 4.2.1 Connection refused
Our servers rate limit all inbound email traffic, in order to protect our customers and our email infrastructure from attacks. This message informs you that the IP address of the mail delivery system has reached its limit by sending too many messages, and that the message may be retried at a later time.
OXSUS00X_142
421 4.2.1 Connection refused
Our servers protect our users by checking the number of recipients your mail delivery system sent messages to over a given time period. This message informs you that your mail delivery system has reached the maximum number of recipients over a given period of time, and that the message may be retried at a later time.
OXSUS00X_201
550 5.7.1 Connection refused
We could not accept your email because your email provider is listed on the Vade Secure Threat Network list.

This list contains IP addresses and FQDNs which have been identified as sending spam. We use this list to protect our subscribers from receiving spam.

OXSUS00X_203
550 5.7.1 Connection refused
We could not accept your email because it failed to pass SPF verification for the sending domain. For more information on troubleshooting this, please refer to https://tools.ietf.org/html/rfc7208.
OXSUS00X_301
550 5.7.1 Connection refused
We could not accept your email because the format of the sender address provided as part of the MAIL FROM command is invalid.
OXSUS00X_303
421 4.2.1 Connection refused
We could not accept your email because the sender domain is invalid or could not be verified: i.e. we were not able to resolve that domain to an A or MX DNS record.
OXSUS00X_304
550 5.7.1 Connection refused
We could not accept your email because the domain used in the From: header either does not match the MAIL FROM address or the SPF record.
OXSUS00X_305
421 4.2.1 Connection deferred
Our servers protect rate limit the number of transactions allowed in a single SMTP session in order to protect our users and infrastructure. The current limit is set to 50 transactions per session. You may retry to deliver the message using a new session.
OXSUS00X_403
550 5.7.1 Connection refused
We could not accept your email because the message was sent to a non-existent customer/user. These errors should be treated as an "unsubscribe" by the sender, if a bulk mailer.
OXSUS00X_404
421 4.2.1 Connection refused
Our servers protect rate limit the number of recipients per message allowed in a single SMTP session over a given period of time in order to protect our users and infrastructure. You may retry to deliver the message at a later time.
OXSUS00X_405
421 4.2.1 Connection refused
We could not accept your email because it exceeds the number of recipients allowed for a single message. The current limit is set to 150 recipients.
OXSUS00X_406
550 5.7.1 Connection refused
We could not accept your email because the format of the recipient's address provided as part of an RCPT TO command is invalid.
OXSUS00X_503
550 5.7.1 Connection refused
We could not accept your email because it does not contain the mandatory From: header, or it was empty.
OXSUS00X_507
550 5.7.1 Message rejected
We could not accept your email because it failed to pass DKIM authentication for the sending domain. For more information on troubleshooting this, please refer to https://tools.ietf.org/html/rfc5585.
OXSUS00X_510
552 5.3.4 Message size limit exceeded
We could not deliver your email because it exceeds the maximum size allowed on our servers, which is currently set to 28MB (29360128 bytes).