Best practices for avoiding high unknown user rates

Maintaining an accurate subscriber database is the foundation of email best practices. Unknown users are invalid addresses in your database.

The unknown user rate is calculated by dividing the number of email addresses associated with unknown user hard bounce messages, or 5xx errors, by the number of email addresses that were attempted.

Some mailbox providers are tolerant of an unknown user rate up to 10%. However, you may see deliverability problems occur with an unknown user rate at or above 5%. Senders with consistently high inbox placement rates keep their unknown user rate below 1%.

Resolving an unknown user problem is a fairly simple process. Prior to sending your next campaign, remove unknown user addresses in your mail logs that hard bounced with a 5xx unknown user error code.

Where to get supporting data

The following data sources can help you evaluate your unknown user rate:

  • Return Path's Real-Time Email Validation
  • Return Path Platform: Sender Score
  • Return Path's Certification program
  • Smart Network Data Services (SNDS)
  • Delivery Log Analysis (DLA)

Best practice recommendations

Address collection methods

  • Reject malformed addresses (for example, me@hotmai.lcom).
  • Reject role accounts such as: abuse@, postmaster@, sales@, customerservice@.
  • Quarantine new data until you send a welcome message and do not receive an unknown user hard bounce (a 5xx error code). This keeps you from adding bad addresses to your regular campaigns.
  • Send a welcome or confirmation message from a separate IP space allowing for bounce processing of invalid addresses and ensuring that high unknown user rates for new subscribers do not impact performance of your regular email campaigns.
  • Send Forward-to-a-Friend messages from a separate IP space to ensure that unknown user rates do not impact the performance of your regular email campaigns.
  • Use CAPTCHA during the subscription process to help prevent list poisoning.
  • Provide an email preference center for easy updating of email addresses.
  • Consider using a double opt-in permission method. Marketers who make customers take an action (usually clicking a link to confirm their subscription) generally have smaller lists than they would otherwise, but those lists are much cleaner than non-confirmed lists. This can help reduce unknown users.

Partner vetting and auditing

If you receive subscriptions from third-party partners, affiliates or list services:

  • Test a sample of the file from a separate IP space and monitor unknown user rates before adding the file to your database.
  • Review the partner data file to ensure that you are not receiving malformed addresses or role account addresses.
  • Mail partners from a separate IP space to monitor their ongoing data quality and unknown user rates.
  • Regularly audit your partner's sign-up process to ensure that it meets industry best practices for address collection.

Unsubscribe processing

  • Reject unsubscribe requests for addresses that are not in your database and prompt the user for the correct email address.
  • Provide an easy method for unsubscribing. Ideally, the request should require only one click.
  • Provide easy update options. Subscribers often change email addresses and may be willing to update their contact information if you make it easy. Even if you do not have a full preference center, offer change of address and frequency options at the point of unsubscribe.
  • Provide multiple venues for subscribers to unsubscribe, including replying to email and preference centers.
  • Allow subscribers to unsubscribe from specific email streams or all email streams using a global unsubscribe function.
  • Provide website confirmation of the address that is requesting the unsubscribe and set expectations about the time frame required to process the request. Ideally, an unsubscribe request should be honored prior to the next email being sent.

Bounce handling

  • Monitor reply-to addresses.
  • Establish hard and soft bounce rules appropriate to your sending practices.
  • Ensure that bounces are appropriately categorized for processing as hard and soft bounces.
  • Monitor unknown user rates by campaign, by data source, and by sending IP address if internal reporting is available.
  • Implement a company-wide suppression list.

Database maintenance

  • Monitor the age of your database.
  • Establish an ongoing process for actively removing subscribers that are both old and inactive.
  • Email your list regularly. In general, the less often you send email, the more likely you are to see high bounce rates. Infrequently emailed lists are also more likely to contain old addresses that were converted into spam trap addresses since your last campaign. Emailing frequently can help reduce unknown users and spam traps.
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