Troubleshooting reputation-based deferrals

Your emails are being deferred based on reputation issues.

Common scenarios
A mailbox provider may defer email for a number of reasons, such as an influx of volume during the holiday season or issues with system capacity. Negative reputation metrics are another common cause of deferrals at an IP address or domain level.

Email may be deferred based on reputation if your IP address or domain has little or no sending history and therefore no reputation.

SMTP logs will help you confirm that a deferral is in place and may help you identify why the mailbox provider is deferring your email. In addition, view all available data sources to identify negative reputation metrics related to bounce codes, time frames, IP addresses, mailstreams, and the domains identified, including the data in: 

  • Certification
  • Return Path Platform
  • Inbox Insight

To determine the cause of deferrals:

  1. Consider the following questions:
  • Are all mailings being deferred or are you only experiencing partial deferrals?
  • Is the deferral widespread or is it related to a particular brand, IP address, sending domain, or mail stream?
  • When did the deferral start? Is it increasing? Is it decreasing?
  • Is the deferral constant or sporadic?
  • Do the deferrals occur on a specific day or time of day?
  • Has this deferral occurred before? If so, could it have been triggered by the same reason?
  • Are you sending from a new IP address or domain?
  1. Review the SMTP bounce logs to identify potential causes.
Contact your email administrator or email service provider to obtain the SMTP logs. Some email service providers may not be able to produce the log files but they may have a report you can run that shows the mailbox provider response codes that look similar to the following examples:
  • 421 RP-001 (COL0-MC3-F15) Unfortunately, some messages from weren't sent. Please try again. We have limits for how many messages can be sent per hour and per day. You can also refer to
  • 421 RP-001: The mail server IP connecting to server has exceeded the rate limit allowed. Reason for rate limitation is related to IP/domain reputation. If you are not an email/network admin please contact your Email/Internet Service Provider for help.
  1. Review how well your sending practices adhere to the mailbox provider's sender guidelines.  
  2. If you are sending from a new IP address and domain, go through a proper warm-up process to establish a good sending reputation.
  3. Confirm that you did not accidentally send to a suppression list.
  4. Ensure you are following best practices for list acquisition and list hygiene.
  5. Validate the receipt and suppression of complaints from the feedback loop.
  6. Analyze the data for:
  • High complaint rates and complaint rate spikes
  • Volume attempted and volume spikes
  • High unknown user rates and unknown user rate spikes
  • A high number of spam trap hits or a spike in the number of hits
  • Engagement metrics, such as recent open and click activity of subscribers, and read rates
  • Timing of the deferrals, such as on a specific day, end of day, end of large send
  • Whitelisting status, if applicable
Although you can resend deferred messages, you should identify the root cause of the deferral first and resolve the underlying issue. Until then, deferrals may continue and your sending queues will likely clog with undelivered and retried messages. If left unresolved, a deferral could escalate into a block.
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