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Troubleshooting mail blocked by a mailbox provider

If your mail is being blocked by a mailbox provider:

1. Review your bounce logs.

A policy block bounce contains a Simple Mail Transfer Protocol (SMTP) error code identifying the reason, which can help you determine the cause of the block. The SMTP error code for a block usually contains a 5XX number, and sometimes a 4XX number, and may look like this:
  • 550 SC-003: Mail rejected by Outlook.com for policy reasons. Your IP address appears to be an open proxy or relay. If you are not an email or network administrator, please contact your mailbox provider for help.
  • AOL 421 DYN: T1 block. This is a temporary block caused by high complaints or low IP reputation.
Policy blocks are almost always related to:
  • A poor IP or domain sending reputation, which may reflect a high complaint rate, high number of unknown users, sending to spam traps, and low engagement
  • Access denied (being in the queue too long)
  • An IP address or domain listed on a blacklist
  • Spam-like characteristics (search for spam or abuse in your bounce logs)
  • Too many connections opened to the receiving server and sending too many emails in a short period of time (connection and throughput)
  • A new IP address or domain
  • Sending to a suppression list
To learn more about the mailbox providers' bounce process and possibly bounce codes, refer to:

2. Assess and troubleshoot the situation.

  • Consider the following questions:
  • Are all mailings being blocked, or is it a partial block?
  • Is the block across all mailbox providers or just one?
  • Is the block widespread or is it isolated to a specific day, date, time, IP address, domain, or mail stream?
  • When did the block start? Is it getting worse or better?
  • Is the block constant or sporadic?
  • Ensure that email was not accidentally sent to a suppression file.
  • Review your complaint handling process and ensure all complainers no longer receive email.
  • Ensure your bounce handling process is working correctly and that unknown users are removed after one to two bounces.
  • Review your list acquisition methods. Did you acquire or purchase new addresses recently from a third party?
  • Review your list hygiene methods to ensure you are sending to people that want to receive your email and engage with you frequently based on your business

3. Review all other data sources about the mailings to identify negative reputation metrics relating to the bounce codes, time frame, IP addresses, mail streams, and domains as described in steps 1 and 2 above.

Sources can include:
  • Your email and engagement data
  • Return Path Certification data
  • Inbox Monitor
  • Reputation Monitor
  • Inbox Insight
  • Microsoft Smart Network Data Services (SNDS) data if the policy block is from Outlook

4. Fix the root cause of the block.

Mailbox providers are less likely to lift a block until they see the sending metrics related to the block improve.
 
5. Contact the mailbox provider through its postmaster site and request the block be lifted.
Be sure to address the root causes of the block; otherwise, the block will not be lifted and it may make it more difficult to get the block lifted in the future.
 
Here is a partial list of mailbox provider support form sites to open a support ticket.
The key to understanding a policy block is to review and use all available data to pinpoint the potential root cause. Blocked email addresses should be removed temporarily and not mailed to until the sender has fixed the issues causing the block. After the cause of block is resolved, the mailbox provider should start accepting email again.
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