How can bounce handling cause email loops?

Bounce handling can create loops through DSN chains:

The loop mechanism:

Message bounces, generating DSN. DSN sent to Return-Path address. If Return-Path is invalid or generates bounce, another DSN created. Cycle continues.

Vulnerable configurations:

Bounce address bounces: Return-Path itself is invalid. Auto-response to DSN: Vacation reply to bounce message. Forward to bad address: Forwarded address bounces back.

Prevention measures:

DSNs should have null Return-Path. Auto-responders should not reply to DSNs. Servers should detect and break loops. Rate limiting on DSN generation.

Standard protections:

RFC requires null sender on DSNs. X-Loop headers track message path. Servers should check for loops before generating DSN.

Bounce loops are echoes that never fade. Proper null envelope handling prevents them.