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Best Practices

Top SMTP Error Messages – June 2016

By Ken Simpson | 5 minute read

In 2012, we posted a list of the world’s most popular SMTP response codes. We are now pleased to present an update to the list. Using a slightly updated script, we

  1. Processed email delivery logs from the MailChannels Cloud SMTP relay service;
  2. Picked up all the SMTP response messages representing permanent delivery failures;
  3. Did a bit of string cleaning to remove serial numbers and other random-looking identifiers; and,
  4. Associated each log line with the organization that generated the error.

Here are the top rejection messages

(organized by the network that generates the error)

Organization Response Code Meaning
GoDaddy ( 550 5.1.0 23.83.x.y is not allowed to send from <> per it’s SPF Record. Please inspect your SPF settings, and try again. IB508  <> The message was rejected because the sender domain ( does not authorize the sending IP address in its SPF record.
552 5.2.0 9qos1t0… – 9qos1t0… This message has been rejected due to content judged to be spam by the internet community IB212 – If you feel this is in error, please submit a request using the following page. <…> GoDaddy’s spam filter thinks the message is spam. By clicking on the provided link, you can report the block as a false positive.
AT&T ( 553 5.3.0 alph135 DNSBL:ATTRBL 521< 23.83.x.y >_is_blocked.__For_information_see_ AT&T’s spam filter has blocked your IP address. You can fill in a form provided by AT&T to request that your IP address be de-listed.
Yahoo ( 554 delivery error: dd This user doesn’t have a account ( [0] – The email address you’re trying to send email to does not exist.
554 delivery error: dd Sorry your message cannot be delivered. This account has been disabled or discontinued [#102]. – The email address you’re trying to send to has been disabled or discontinued.
554 5.7.9 Message not accepted for policy reasons.  See Yahoo! has rejected your message for some other reason.
554 delivery error: dd This account has been temporarily suspended. Please try again later. – Yahoo has temporarily disabled email for the recipient you are trying to reach. This may happen if an email recipient receives a great deal of spam.
Endurance International Group ( 550 <> Sender rejected. A bounce generated by your mail server has been rejected by the receiving server to which the bounce was addressed.
Google ( 550 5.7.1 Unauthenticated email from is not accepted due to domain’s

550-5.7.1 DMARC policy. Please contact administrator of domain if

550-5.7.1 this was a legitimate mail. Please visit

550-5.7.1 to learn about DMARC

550 5.7.1 initiative. s140si61565….111 – gsmtp

Google’s mail server has rejected your message because the domain of the sender address ( in this example) has a DMARC policy that you have somehow violated. DMARC enables domain owners to control how email from their domain may be sent. For more information, visit the link provided by Google.
552 5.7.0 This message was blocked because its content presents a potential

552-5.7.0 security issue. Please visit

552-5.7.0 review our message

552 5.7.0 content and attachment content guidelines. c133si5731….145 – gsmtp

Google’s mail server has rejected the message because it contains a file attachment that is of a “high risk” type. At present, Google prohibits the following types of attachments:


552 5.2.3 Your message exceeded Google’s message size limits. Please visit

552-5.2.3 to view our size

552 5.2.3 guidelines. 198si450….22 – gsmtp

Google does not accept attachments that are larger than 25MB in size. In some situations, smaller attachments may also be rejected.
550 5.7.1 [23.83.x.y       7] Our system has detected that this message is

550-5.7.1 likely unsolicited mail. To reduce the amount of spam sent to Gmail,

550-5.7.1 this message has been blocked. Please visit

550 5.7.1 more information. x8si4242….230 – gsmtp

Google’s spam filter has determined your message is spam, and has decided to reject it rather than put it in the recipient’s spam folder.
550 5.7.1 [      11] Our system has detected that this message is

550-5.7.1 not RFC 5322 compliant. To reduce the amount of spam sent to Gmail,

550-5.7.1 this message has been blocked. Please review

550 5.7.1  RFC 5322 specifications for more information. vw4si4702….180 – gsmtp

Google blocks messages which are not strictly compliant with the RFC 5322 message format.
550 5.7.1 Please update the address to and try again. – gcdp j9si4733….36 – gsmtp The recipient you’re trying to reach has moved to a new domain. To fix this problem, you need to update the recipient address to use the new domain suggested (in this example, “”).
Microsoft (, 550 5.7.0 (COL004-MC2F47) Message could not be delivered. Please ensure the message is RFC 5322 compliant. Microsoft insists that your message be fully compliant  with the RFC 5322 message format.
550 5.7.0 (BAY004-MC5F13) Unfortunately, messages from (23.83.x.y) on behalf of ( could not be delivered due to domain owner policy restrictions. Your message was rejected because the sender’s domain name (e.g. does not permit delivering email from your mail server. Use the SPF record testing tool to find out why.

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