SMTP Enhanced Status Codes
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Three-digit dotted codes (e.g., 5.1.1) defined in RFC 3463 that provide more specific diagnostic information than basic three-digit SMTP reply codes. The first digit matches the SMTP reply class (2 = success, 4 = transient failure, 5 = permanent failure), the second identifies the subject (0 = other, 1 = addressing, 7 = security), and the third provides detail. Example: 5.1.1 means the recipient address does not exist.