How do enhanced status codes (X.Y.Z) work?

Enhanced status codes provide detailed diagnostic information:

Format: X.Y.Z

X (Class): 2=Success, 4=Temporary failure, 5=Permanent failure.

Y (Subject): 0=Other, 1=Addressing, 2=Mailbox, 3=Mail system, 4=Network, 5=Protocol, 6=Content, 7=Security.

Z (Detail): Specific condition within the subject category.

Common examples:

5.1.1: Bad destination mailbox address. 5.2.2: Mailbox full. 5.7.1: Delivery not authorized, message refused. 4.7.0: Temporary authentication failure.

Usage:

Appear alongside or after basic SMTP codes. Provide machine-readable detail. Enable more precise bounce classification.

Enhanced codes are the fine print. They tell you exactly what went wrong beyond the headline.