Glossary: Logs and Data Sources for Investigations

Mail reply codes

Also written: 550, 550 5.1.1, dsn=2.0.0, mail status codes

Codes a mail server records or sends back for each message: 550 means a permanent failure, 5.1.1 means the recipient address doesn't exist, and 2.0.0 means delivered. Postfix used 550 5.1.1 to reject the phishing email sent to the nonexistent payroll@kettleandcrane.com.