It's time to decode another deliverability acronym. Today, we're going to tackle NDR, which stands for "Non-Delivery Report."

NDR is another way to say "bounce message." I consider them essentially to be synonyms. In my opinion, the acronym NDR is a bit of a Microsoft-ism, and the more appropriate generic term is probably "bounce message."

What's a bounce message? Whenever you try to email somebody, but you get a notification email back that tells you that your attempt to mail that someone was unsuccessful. That response, notifying you of the delivery failure is an NDR or bounce message.

When you use an email service provider, CRM or newsletter tool to send email messages, these tools typically receive and process any bounces for you automatically. It's not always necessary to generate a whole NDR email message when a message doesn't go through -- much of the time, when a newlsetter tool or other bulk email platform attempts to deliver a mesasge, but is unable to, that rejection happens "inline" during the SMTP communication between sending and receiving server. The sending server often just logs the rejection directly in their system, without generating a notification email back to the person who initiated the email send. If these email tools did not process bounces for the sender, those sending large enough volumes of bulk email or newsletters would find themselves inundated with bounce messages after each email send, and the sender would have to choose wether or not to manually suppress or unsubscribe each bouncing address. This makes automatic bounce processing a significant time saver for those sending lots of email messages.

"What percentage of a sender's mail is bouncing" can sometimes be used as a metric to determine how spammy a sender is (or isn't). As email addresses churn over time, addresses not emailed in a long time, or obtained via methods other than opt-in permission, have a higher likelihood of no longer being valid by the time a sender attempts to mail to them. This is a bit of an oversimplification as the whole equation can get a bit complex, but the short version is, if a high percentage of a list is getting bounced back with "user unknown" rejections, that sender likely has a list hygiene issue and is likely to have deliverability issues.

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