Koltrix
Guides
Ports 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
DeliverabilityDANE vs MTA-STS for inbound mail: the trade-offs
Both stop STARTTLS downgrades, with different trust models. Compare DANE's DNSSEC dependency with MTA-STS's web PKI approach and decide what to deploy.
5 min read
- Guides
MTA-STS from scratch: DNS record, policy file, testing
Set up MTA-STS for inbound mail: the _mta-sts TXT record, the HTTPS policy file, testing mode, then enforce, plus the mistakes that break delivery.
5 min read
DeliverabilityTLS-RPT: get told when inbound TLS fails
SMTP TLS Reporting (RFC 8460) sends you daily JSON summaries of failed TLS sessions. How to publish the record and read the reports that come back.
5 min read