Email Bounce Classification: Understanding NDR Codes and What They Mean
Non-delivery reports contain valuable diagnostic information. Learn to read bounce codes, classify bounce types, and use bounce data to improve your email deliverability.
Marcus Webb
Email Marketing Specialist
Every bounced email generates a Non-Delivery Report with diagnostic codes explaining the failure. These codes follow SMTP response codes and DSN formats that tell you exactly what went wrong. Learning to read them transforms bounces from frustrating failures into actionable diagnostic data.
SMTP response codes are three digits. 2xx indicates success, 4xx persistent temporary failure (soft bounce), and 5xx permanent failure (hard bounce). Common codes include 550 for mailbox unavailable, 551 for invalid address, 552 for mailbox full, and 554 for transaction failed.
“DSN codes provide granular classification with number.number.number format. The first number indicates class. The second indicates subject: 1 for addressing, 2 for mailbox, 5 for delivery protocol, 6 for content, 7 for security. For example, 5.1.1 is a permanent invalid address failure.
Implement automated bounce processing based on code classification. Permanently remove 5.x.x addresses immediately. Retry 4.x.x codes up to three times before suppression. Flag 7.x.x security codes for human investigation. Automated processing removes invalid addresses within minutes.
Bounce data analysis reveals larger patterns affecting deliverability. Track rates by sending IP, domain, date, and campaign type. Elevated bounce rates from a particular provider may indicate changed acceptance policies. Spikes on particular days may indicate configuration changes.
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