A “bounced e-mail message” (or simply “bounce”) is a message that cannot be delivered to its destination. Usually, Internet Service Providers (ISPs) will return the e-mail to the sender along with some information indicating why the message could not be delivered.

Different kinds of bounces can be grouped depending on whether the fault lies with the sender, the receiver, or somewhere in between. When it handles bounces due to problems with the recipient, it may invalidate the recipient’s e-mail address. This cleans mailing lists and segments by preventing any future mailings from being sent to that recipient, this is an important step in maintaining a good sender reputation. When Carma handles bounces due to problems with the sender or problems in transit, it takes no automatic action. For all bounces, regardless of type, it records as much information as possible for reporting.

When ISPs see a high unknown user rate from an IP they may suspect a “dictionary attack” is occurring or that the sender does not have solid data hygiene practices. A dictionary attack is when a spammer uses combinations of words in the dictionary to create random combinations of emails addresses and then deploys spam to those addresses for the purpose of finding valid email addresses. These addresses are then collected, sold and sent more spam. ISPs do not like this because processing all these unknown users is a hardship to their infrastructure.

Hard bounced vs Soft bounce

  • A hard bounce is a recipient problem that is permanent and is not expected to clear up. The classic cause of a hard bounce is a destination address that does not exist due to a typo or account termination. When Carma encounters a hard bounce, it immediately invalidates the contact’s e-mail address to prevent further mailings from being sent.
  • A soft bounce is a recipient problem that is temporary and may clear up at some point in the future. The classic cause of a soft bounce is a full mailbox – the message cannot be delivered now, but in the near future it is possible that either the storage quota will be lifted or the mailbox will be cleared out. You will still be able to send to addresses that soft bounced.

Bounce categories

There are hundreds of different bounce types and it often seems that depending on the ISP they have different messages they send back to Carma for us to understand why the email bounced. To make it easier for you as a user to understand why your email bounced we group them into different categories. For more information about bounce categories, click here.

When do Carma stop sending to bounced addresses?

  1. When Carma encounters a hard bounce, it immediately invalidates the contact’s e-mail address
  2. If an email address gets 5 soft bounces in a row Carma will convert the bounces to a hard bounce and will set the recipient as inactive. This is primarily to avoid increased problems with deliverability which will occur when you keep on sending emails to recipients that only reports as soft bounced over and over again. (Main reason to this is that receiving server is incorrectly categorizing a hard bounce as a soft bounce.)

How do I prevent future bounces?

  1. Make sure to check the bounce statistics for all emails, just as often as you check the open rate.
  2. Use Google’s reCaptcha  Read all about it here.
  3. Sending a “Welcome” or “Confirmation” message from a separate IP space allows for bounce processing of invalid addresses and ensures that high unknown user rates for new subscribers do not impact performance of regular email campaigns sent from different IPs.
  4. On sign-up pages, ask customers to type in the email address twice to prevent “fat fingering” mistakes which could unintentionally result in an address that does not exist.
  5. When receiving 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 the database.
    • Review the partner data file to ensure that malformed, role account and nonsensical email addresses are removed.
    • Mail partners data from separate IP space to monitor their on-going data quality and Unknown User rates.
    • Regularly audit partner’s sign-up process to ensure that it meets industry best practices for address collection.
  6. Establish an on-going process for actively removing subscribers that are both old and inactive.


Next: Complaints

This is an article in the Carma Campus Class in Deliverability