What is a “custom” SMTP code?

A custom SMTP code extends beyond the standard RFC definitions:

What they are:

Proprietary codes created by specific mail providers. Used to convey detailed information not covered by standards. May use standard code numbers with custom text. May use enhanced status codes with custom meanings.

Examples:

Microsoft uses codes like "421 RP-001" for rate limiting. Gmail includes custom reason strings in standard codes. Yahoo uses specific messages for policy violations.

Challenges:

Require provider-specific documentation to interpret. May change without notice. Complicate bounce parsing across providers.

Response:

Consult provider-specific documentation. Build flexible bounce parsing that handles variations. Track patterns for common custom codes.

Custom codes are local dialects of SMTP. You need to learn each harbor's specific signals.