Overview

Bounce Classification is a Mailgun Optimize feature that’s powered by Mailgun sending data. It classifies logs from your Mailgun sending to identify high impact bounces that require the sender's attention. Issues identified by bounce classification should be addressed so that they do not degrade your sending reputation. Keep an eye out for increases in classified bounces to resolve deliverability issues as quickly as possible.

Setup

All domains sending through Mailgun will have their bounces classified, but viewing detailed data on bounces requires the domain to be monitored with Mailgun Optimize. To setup bounce classification, first add your Mailgun sending domain(s) to domain monitoring in Mailgun Optimize.

Bounce Reasons

We currently classify four different bounce reasons: blocklistings, reputation blocks, content blocks, and authentication blocks. If left unaddressed, each of these types of bounces can significantly impact your reputation.

Blocklistings

Blocklisting classifications indicate your IP or domain has been added to a blocklist and your mail is being bounced or rejected. We classify blocklistings from major blocklist providers as well as from mailbox providers with internal blocklists. When blocklisted, it’s recommended that you pause sending from the effected domains and assigned IPs until you can identify the root cause of the blocklisting, put mitigation in place, and achieve delisting.

Blocklistings from Spamhaus, Barracuda, Invaluement, Cloudmark, Spamcop, AT&T, Comcast, and Microsoft will be detected by Bounce Classification.

Spamcop blocklisting:

Service unavailable; Client host [example.com] blocked by 
bl.spamcop.net; xx.xx.xx.x (example.com:blocked)

Reputation Blocks

If your sending reputation falls too low with a mailbox provider, they may start rejecting your email. Continuing to send while blocked will further degrade your reputation. We recommend you pause sending from the effected domain/IP and investigate what may have caused a reputation decrease, so that you can put the appropriate mitigation efforts in place.

Reputation blocks from Gmail, Yahoo, and Outlook will be detected with bounce classification.

Gmail reputation block:

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

Content Blocks

Content blocks identify an issue with the content of an email. This includes attachments and URLs on the email. If you see these errors, you should identify the content that’s resulted in these bounces and modify the content to comply with mailbox provider requirements.

Gmail content block:

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.

Authentication Blocks

To assist customers with the recent Google and Yahoo DMARC authentication requirements, we now classify DMARC bounces from those two providers, as well as several DKIM and SPF bounces. If you receive these bounces, please verify that your DMARC, DKIM, and SPF records are up to date.

Gmail DMARC bounces:

4.7.26 Unauthenticated email from <domain.tld> is not accepted due to 
domain's DMARC policy, but temporary DNS failures prevent authentication. 
Please contact the administrator of <domain.tld> domain if this was 
a legitimate mail. To learn about the DMARC initiative, go to 
https://support.google.com/mail/?p=DmarcRejection - 

5.7.26  https://support.google.com/mail/?p=DmarcRejection - gsmtp

Yahoo DMARC bounce:

5.7.9 Message not accepted for policy reasons. See 
https://postmaster.yahooinc.com/error-codes

Investigating Further

If you experience classified bounces and would like to dig deeper, follow these steps to view the logging associated with an individual classified bounce:

1. Navigate to the Bounce Classification feature. You can use this direct link. 
2. Select a listed domain with bounces in the Bounce Classification.
3. Expand the available mailbox providers that generate classified bounces.
4. Click the Occurrences link found on the right side of any classified bounce row.
   
5. View all classified logs and expand the logs to see further details as necessary.
   

Need Support?

Our Support Team here at Sinch Mailgun is happy to help! Reach out to us in the Support page of your Mailgun Control Panel, and we'll be with you shortly!