Koltrix
Email API
Retry policies for outbound mail: backoff and give-up times
How long should a sender keep retrying a deferred message? Exponential backoff with jitter, sensible give-up windows and what to tell users meanwhile.
4 min read
GuidesTesting email locally with a fake SMTP server
Point your app at a local SMTP catcher during development to see every message without risking real delivery. Setup, workflow and what it cannot test.
4 min read
GuidesPorts 25, 465, 587 and 2525: which should you use?
Port 25 is for server-to-server relay, 587 and 465 are for submission, and 2525 is a common fallback. What each port expects and why clouds block 25.
4 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
GuidesMIME for developers: alternative, related and mixed
Most email bugs with images and attachments are MIME tree bugs. Learn how multipart/alternative, related and mixed nest, with a correct example.
5 min read
Email APIHard 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 APIPer-provider throttling: pacing mail to big mailbox providers
Gmail, Outlook and Yahoo each defer differently when you send too fast. Build per-destination queues and adaptive concurrency that respond to 4xx replies.
4 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
Email APISMTP relay vs HTTP API: how should your app send email?
SMTP relays are universally compatible; HTTP APIs are easier to make idempotent and observable. Compare them on errors, retries, security and speed.
4 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
- Guides
Testing SMTP by hand with swaks and openssl
Talk to a mail server directly to see what it really says. Use swaks and openssl s_client to test auth, TLS, relaying and rejections step by step.
5 min read
Email APIUnicode 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