Koltrix
Guides
Tracing a delivery through Received headers
Received headers record every hop a message took, newest first. Read them bottom-up to find delays, rewrites and the server that touched your mail.
5 min read
Email APIImplementing one-click unsubscribe (RFC 8058) correctly
One-click unsubscribe needs two headers, a DKIM signature that covers them, and an endpoint that accepts a POST without a login. Build it step by step.
5 min read
Email APIMessage-ID, In-Reply-To and References for app email
Control how notification emails thread in Gmail and Outlook with Message-ID, In-Reply-To and References, and how to break unwanted threads apart.
4 min read
- Email API
Unicode in email: SMTPUTF8, encoded headers and IDNs
Non-ASCII names, subjects and addresses need different mechanisms. How RFC 2047 encoded words, SMTPUTF8 and IDN domains fit together in practice.
4 min read
- Guides
Reading Authentication-Results headers like a receiver
The Authentication-Results header records the SPF, DKIM and DMARC verdicts a receiver reached. Decode each field and use it to debug failures fast.
5 min read