Koltrix
Deliverability
SPF flattening: when it helps and when it breaks mail
Flattening replaces includes with IP ranges to dodge the lookup limit. Here is how it works, why it rots, and safer alternatives like subdomains.
5 min read
Blog · Deliverability · page 3 of 5
Deliverability
SPF, DKIM, DMARC, BIMI, sender reputation and the inbox rules of Gmail, Yahoo and Microsoft: what decides whether your email arrives.
DeliverabilityWarming up a brand-new sending domain
A new domain has no reputation, which filters treat with suspicion. Authenticate first, start with your most engaged recipients and grow steadily.
5 min read
DeliverabilityEd25519 DKIM: should you dual-sign yet?
RFC 8463 added Ed25519 to DKIM, but receiver support is uneven. Learn how dual signing with RSA works and whether it is worth the operational cost.
4 min read
DeliverabilitySPF ~all vs -all: what each qualifier does in practice
Softfail or hard fail? How receivers treat ~all and -all today, why DMARC changed the calculus, and which ending to pick for each kind of domain.
4 min read
DeliverabilityThe SPF 10-lookup limit: how to count and get back under it
Which SPF mechanisms count toward the 10-lookup limit, how to audit your record by hand, and the safe ways to trim includes before you hit permerror.
5 min read
DeliverabilityDKIM header oversigning and why the l= tag is a trap
Which headers to sign, why signing From twice blocks header injection, and how the l= body length tag lets attackers append content to signed mail.
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
DeliverabilityGetting off a blocklist: a calm, step-by-step playbook
Before requesting removal from a blocklist, find and fix what got you listed. A sequenced playbook for confirming, remediating and delisting.
4 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
DeliverabilityThe 0.3% spam rate line: how it is measured and avoided
Gmail and Yahoo both draw a line at a 0.3% spam complaint rate and recommend staying under 0.1%. How the rate is calculated and how to keep it low.
4 min read
DeliverabilityA staged path from DMARC p=none to p=reject
A week-by-week DMARC rollout: monitor with p=none, fix every legitimate source, move to quarantine, then reject, with clear exit criteria per stage.
4 min read