What are some known specific bounce messages from Gmail?

Gmail provides detailed bounce messages:

Authentication-related:

"550-5.7.26 This message does not have authentication information or fails to pass authentication checks." "550-5.7.1 Messages missing a valid DKIM signature."

Spam-related:

"550-5.7.1 Our system has detected that this message is likely spam." "421-4.7.0 Our system has detected an unusual rate of unsolicited mail."

Rate limiting:

"421-4.7.0 Try again later." "452-4.5.3 Domain policy size per transaction exceeded."

User-related:

"550-5.1.1 The email account that you tried to reach does not exist." "552-5.2.2 The recipient's inbox is full."

Characteristic features:

Multi-line responses (550-5.7.1). Support.google.com references. Specific enhanced status codes.

Gmail tells you exactly what went wrong. Read the full message for actionable guidance.