Make the Suppression System More Agile

Summary Enable the Simple Mail Service suppression system to differentiate between temporary, permanent, and internal errors to avoid overly aggressive blocking of email addresses. Current Situation The Simple Mail Service automatically suppresses sending messages to email addresses that have too many delivery errors to reduce delivery error rates and improve domain reputation and deliverability. However, the current implementation appears to be overly aggressive in its suppression logic. Problem Statement The suppression system currently blocks email addresses for extended periods after even a single error, regardless of the error type. This includes: Temporary errors - Where the recipient could successfully receive the email if retried shortly after Internal formatting errors - Such as RFC compliance issues that have nothing to do with the recipient's email address validity Specific Issues Identified Issue 1: Aggressive Blocking on Single Errors The system blocks addresses for extended periods after a single failure, even when the error is neither permanent nor external. In many cases, customers could successfully retry sending the email shortly after the initial failure. Issue 2: Suppression Due to Email Formatting Errors Suppressions can be triggered by errors in the email message itself that are unrelated to the receiving email address. For example: Adding an incorrect or duplicate "To" header results in a bounce and suppression RFC 5322 compliance errors (such as multiple To headers) cause Gmail to block the message, triggering suppression of the recipient address Example error: "This message is not RFC 5322 compliant. There are multiple To headers. To reduce the amount of spam sent to Gmail, this message has been blocked." Proposed Solution Implement a more intelligent suppression system that: Categorizes error types: Temporary errors (retry-able) Permanent errors (hard bounces) Internal/formatting errors (sender-side issues) Applies different suppression strategies based on error type: Temporary errors: Short suppression period or immediate retry capability Permanent errors: Current suppression behavior Internal errors: No suppression of the recipient address Prevents recipient blocking for sender-side issues: RFC compliance errors Malformed headers Other formatting issues Expected Outcome A more nuanced suppression system that maintains high deliverability standards while avoiding unnecessary blocking of valid email addresses due to temporary or internal errors.