Why do different ISPs use non-standard codes?

ISPs deviate from standard codes for several reasons:

More granular information:

Standard codes are broad categories. Custom codes provide specific diagnostic details. Helps sophisticated senders troubleshoot issues.

Proprietary features:

Reputation systems unique to that provider. Custom rate limiting or throttling logic. Specific policy enforcement mechanisms.

Security through obscurity:

Less detail to spammers about what triggered rejection. Custom formats harder to automate against.

Legacy and evolution:

Systems built before current standards. Incremental additions over time. Backwards compatibility constraints.

Implications for senders:

Need provider-specific bounce handling. Documentation may be incomplete. Trial and error sometimes necessary.

Each ISP is a sovereign port with its own rules. Learn the local customs for each destination.