Do mailbox providers use custom bounce codes or messages?

Major mailbox providers extensively use custom codes and messages:

Why providers customize:

Standard codes are too generic. Custom messages aid troubleshooting. Proprietary systems need proprietary signals. Detailed feedback helps legitimate senders.

Examples by provider:

Gmail: Detailed text with support.google.com references. Microsoft: Alphanumeric codes like "RP-001" for rate limiting. Yahoo: TS01, TS02 style throttling codes. Proofpoint, Barracuda: Gateway-specific messages.

Challenges for senders:

Must maintain provider-specific parsing. Documentation may be incomplete. Custom formats change without notice.

Best practice:

Build flexible bounce parsing. Reference provider postmaster documentation. Track patterns from your own sending.

Each major provider speaks a distinct dialect. Learn the local language for each destination.