What is a “non-standard bounce” (custom message)?

A non-standard bounce deviates from RFC specifications:

What makes it non-standard:

Custom SMTP codes beyond RFC definitions. Proprietary message formats. Provider-specific reason strings. Unique error identifiers.

Examples:

Microsoft's "RP-001" rate limiting codes. Gmail's detailed policy messages. Custom spam filter responses. Legacy system quirks.

Challenges:

Requires provider-specific parsing. Documentation may be incomplete. Can change without notice. May not fit standard categories.

Handling approaches:

Build provider-specific rules. Fall back to conservative classification. Track and analyze unknown patterns. Update dictionaries as patterns emerge.

Non-standard bounces are local customs. Each port has rules not in the international guidebook.