Provider-specific Bounce Codes & Messages

Not all "harbors" use the same codes. This section explains how Gmail, Outlook, and Yahoo have their own custom bounce messages that provide extra clues about why they blocked you (e.g., "Message rejected due to user complaints").

15 questions in this topic

Do mailbox providers use custom bounce codes or messages? What are some known specific bounce messages from Gmail? What are some known specific bounce messages from Outlook? How can I interpret provider-specific bounce details? How do Gmail, Yahoo, and Outlook label bounces differently? What are Gmail “reason codes”? How does Microsoft classify errors (5.1.0, 5.2.2, 5.7.1)? How do Yahoo “deferred due to policy” messages appear? How do Zoho and ProtonMail handle unknown users? What is “421 RP-001” (Microsoft rate limit)? What are “451 4.7.1” throttling messages? What is “temporarily deferred due to user complaints”? How does Postfix format DSNs? How do ESPs like SendGrid or Cakemail standardize bounce data? How do internal vs external MTAs treat the same error code?

Questions about Provider-specific Bounce Codes & Messages