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smtp response codes

SMTP Codes

SMTP Error Codes: 4xx and 5xx Reply Codes Explained

Alaa 18 September 2026

This directory documents 93 SMTP error codes, from common 421 soft bounces to obscure server-specific 554 rejections from Nemesis, Comcast cmsmtp, GoDaddy osplibsmtp, and other production MTAs. Each ...