Koltrix
Email API
Processing bounce and complaint webhooks end to end
Turn bounce and complaint events into suppressions, user-facing flags and alerts. A worked pipeline from webhook receipt to updated account state.
5 min read
DeliverabilityGmail's stricter enforcement: reading its 4xx and 5xx replies
Since late 2025 Gmail rejects more non-compliant mail outright. How to read Gmail's SMTP replies, separate temporary from permanent, and fix the cause.
5 min read
- Email API
Hard vs soft bounces: classifying SMTP replies correctly
A 5xx is not always permanent and a 4xx is not always harmless. Build bounce classification on reply codes and enhanced status codes, with exceptions.
4 min read
Email APISuppression lists: what to suppress and for how long
Hard bounces, complaints and unsubscribes each deserve different suppression rules. Design a suppression list, scopes and expiry that protect reputation.
5 min read
GuidesSMTP enhanced status codes (RFC 3463), decoded
Enhanced status codes like 5.1.1 and 4.7.0 say why delivery failed. Learn the class.subject.detail structure and the codes you will see most often.
5 min read
- Deliverability
Greylisting and 4xx deferrals: why your retries matter
Greylisting temporarily rejects unknown senders and expects a retry. How it works, what deferrals look like in logs and the retry behavior receivers expect.
4 min read