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Overview
Mailgun Optimize's Bounce Classification feature helps to identify critical bounces that lower your email reputation and cause other problems with your sending. The feature not only classifies these bounces by sending domain and mailbox provider/spam filter but also provides the relevant error message as well as recommended solution.
As such, we term a bounce processed by this feature as a Classified Bounce. Because the email landscape is always changing, the bounces we classify are also dynamic. This article will provide you with the most up-to-date information regarding the current list of classified bounces.
List of Classified Bounces
Below is a list of all Classified Bounces currently processed by our Bounce Classification feature.
| Mailbox Provider / Spam Filter |
Category |
Code |
Sample Error Message |
| Apple | ESPBLOCK | 554 | 5.7.1 [CS01] Message rejected due to local policy. Please visit https://support.apple.com/en-us/HT204137 |
| Apple | BLACKLISTED | 550 | smtp;550 5.7.1 Your email was rejected due to having a domain present in the Spamhaus DBL -- see https://www.spamhaus.org/dbl/ |
| ATT | BLACKLISTED | 553 | 5.3.0 flpd568 DNSBL:RBL 521< x.xx.xx.xx >_is_blocked.For assistance forward this error to abuse_rbl@abuse-att.net |
| Barracuda | BLACKLISTED | 554 | smtp; 554 rejecting banned content |
| Cloudmark | BLACKLISTED | 554 | smtp; 554 mcc-ibgw-6001a.ext.cloudfilter.net cmsmtp [x.xx.xx.xx] is listed on Cloudmark CSI-Global. Please visit https://csi.cloudmark.com/en/reset AUP#RBL |
| Cloudmark | BLACKLISTED | 550 | smtp; 550 5.7.1 Service unavailable; client [x.xx.xx.xx] blocked using Cloudmark Sender Intelligence (Visit https://csi.cloudmark.com/reset-request/ if you feel this is in error.). Please provide the following IP address when reporting problems: [x.xx.xx.xx] |
| Cloudmark | BLACKLISTED | 550 | smtp; 550 5.7.1 Service unavailable; client [x.xx.xx.xx] blocked using Proofpoint Dynamic Reputation (Visit https://ipcheck.proofpoint.com/ if you feel this is in error.). |
| Comcast | BLACKLISTED | 554 | [x.xx.xx.xx] found on one or more DNSBLs, see https://postmaster.comcast.net/smtp-error-codes.php#BL000100 |
| Comcast | ESPBLOCK | 554 | 5.1.0 xx.xx.xx.x Comcast block for spam. Please see https://postmaster.comcast.net/smtp-error-codes.php#BL000000 |
| EOP (Exchange Online Protection) | BLACKLISTED | 550 | smtp; 550 5.7.1 Service unavailable, Client host [x.xx.xx.xx] blocked using Spamhaus. To request removal from this list see https://www.spamhaus.org/query/ip/x.xx.xx.xx (AS3130). |
| EOP (Exchange Online Protection) | BLACKLISTED | 550 | smtp; 550 5.7.606 Access denied, banned sending IP [x.xx.xx.xx]. To request removal from this list please visit https://sender.office.com/ and follow the directions. For more information please go tohttp://go.microsoft.com/fwlink/?LinkID=526655 AS(1430) |
| Free | ESPBLOCK | 550 | Too many spams from your IP (x.xx.xx.xx), please visit http://postmaster.free.fr/ |
| Gmail | ESPBLOCK | 550 | smtp;550 5.7.1 [x.xx.xx.xx] Our system has detected that this message is likely suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mail/answer/ for more information. - gsmtp |
| Gmail | ESPBLOCK | 550 | smtp;550 5.7.1 [x.xx.xx.xx] Our system has detected that this message is likely suspicious due to the very low reputation of the sending IP. To best protect our users from spam, the message has been blocked. |
| Gmail | ESPBLOCK | 550 | smtp;550 5.7.1 [x.xx.xx.xx] Our system has detected that this message is likely unsolicited mail. To reduce the amount of spam sent to Gmail, this message has been blocked. Please visit https://support.google.com/mail/?p=UnsolicitedMessageError for more information. - gsmtp |
| Gmail | ESPBLOCK | 421 | 421 4.7.28 Gmail has detected an unusual rate of unsolicited mail originating from your DKIM domain. To protect our users from spam, mail sent from your domain has been temporarily rate limited. Please visit https://support.google.com/mail/?p=UnsolicitedRateLimitError to review our Bulk Email Senders Guidelines. |
| Gmail | ESPBLOCK | 552 | 5.7.0 This message was blocked because its content presents a potential security issue. Please visit https://support.google.com/mail/?p=BlockedMessage to review our message content and attachment content guidelines. |
| Gmail | ESPBLOCK | 550 | 5.7.26 https://support.google.com/mail/?p=DmarcRejection - gsmtp |
| Gmail | GENERIC | 421 |
4.7.26 This mail has been rate limited because it is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM. Authentication results: DKIM = did not pass SPF [example.com] with ip: [xx.xx.xx.x] = did not pass For instructions on setting up authentication, go to https://support.google.com/mail/answer/81126#authentication - gsmtp |
| Gmail | GENERIC | 421 |
4.7.27 This mail has been rate limited because SPF does not pass. Gmail requires all large senders to authenticate with SPF. Authentication results: SPF [domain.com] with ip: [xx.xx.xx.x] = did not pass For instructions on setting up SPF authentication, go to https://support.google.com/a/answer/10685031 - gsmtp |
| Hotmail | BLACKLISTED | 550 | 5.7.1 Service unavailable, MailFrom domain is listed in Spamhaus. To request removal from this list see https://www.spamhaus.org/query/lookup/ |
| Hotmail | ESPBLOCK | 550 | 5.7.1 Unfortunately, messages from [x.xx.xx.xx] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3140). You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. |
| Invaluement | BLACKLISTED | 550 | Connection refused. RBL [IP-x.xx.xx.xx] Blocked by Invaluement - see https://www.invaluement.com/research/ |
| Outlook | ESPBLOCK | 451 | 451 4.7.650 The mail server [x.xx.xx.xx] has been temporarily rate limited due to IP reputation. For e-mail delivery information, see https://postmaster.live.com (S843)[#.prod.protection.outlook.com] |
| Outlook | ESPBLOCK | 550 | smtp;550 5.7.1 Unfortunately, messages from [x.xx.xx.xx] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150). You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. [#.eop-nam02.prod.protection.outlook.com] |
| Spamcop | BLACKLISTED | 550 | Service unavailable; Client host [example.com] blocked by bl.spamcop.net; xx.xx.xx.x (example.com:blocked) |
| Yahoo | ESPBLOCK | 554 | smtp;554 Message not allowed - [PH01] Email not accepted for policy reasons. Please visit https://postmaster.yahooinc.com/error-codes |
| Yahoo | ESPBLOCK | 553 | smtp; 553 5.7.1 [TSS07] Connections will not be accepted from [x.xx.xx.xx], because the ip is in Spamhaus's list; see https://postmaster.yahooinc.com/error-codes |
| Yahoo | ESPBLOCK | 554 | 5.7.9 Message not accepted for policy reasons. See https://postmaster.yahooinc.com/error-codes |
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