Koltrix
Guides
DNS TTL strategy for email authentication changes
Lower TTLs before you change SPF, DKIM or MX, and know which caches ignore you. A practical timeline for authentication changes without surprises.
5 min read
DeliverabilityReverse DNS for mail servers: PTR, FCrDNS and HELO
Receivers expect a sending IP to have a PTR record that resolves back to the same IP and a sane HELO name. Configure all three and verify them.
4 min read
Guidesdig commands every email engineer should know
The dig queries that answer most email questions: SPF, DKIM selectors, DMARC, MX, PTR, MTA-STS and BIMI, plus how to bypass caches when testing.
5 min read
DeliverabilityDMARCbis is now RFC 9989: what changes for your record
DMARC moved to the standards track as RFCs 9989, 9990 and 9991. Here is what changed: pct is gone, np and psd arrive, and a DNS tree walk replaces the PSL.
4 min read
DeliverabilityDangling DNS records and email: subdomain takeover risks
Stale CNAMEs and forgotten includes can let someone else send or host content on your domain. How takeovers happen and how to audit DNS for them.
5 min read
GuidesEmail Migration Rollback Plan: When the Cut-Over Goes Wrong
Most email migrations go fine, but the bad ones go badly fast. A rollback plan: triggers, the DNS values to save, and how to reconcile mail after reverting.
5 min read
GuidesLeaving Web-Host Email (cPanel) for a Real Business Inbox
The email bundled with your web hosting was fine to start with. Why it causes trouble as you grow, and how to move mail off cPanel without breaking your site.
5 min read
GuidesMigrating Email Away From Zoho Mail, Step by Step
How to move a domain's mail off Zoho Mail without losing messages: export options, recreating aliases and groups, DNS changes, and checking every integration.
5 min read
GuidesMoving Off Microsoft 365: An Email Migration Plan
A step-by-step plan for moving a domain's email off Microsoft 365: inventory, exporting mailboxes, DNS records to change, coexistence, and what to check after.
5 min read
DeliverabilityDMARC for subdomains: sp=, np= and domains you forgot
Attackers love subdomains you never configured. How sp= and the new np= tag cover them, how policy discovery works, and when to publish per-subdomain records.
4 min read
GuidesEmail Setup for Agencies Managing Many Client Domains
A practical system for agencies that touch email on dozens of client domains: who owns DNS, how to get access, what to record, and how to offboard cleanly.
5 min read