Koltrix
Deliverability
Gmail and Yahoo bulk sender rules: the full checklist
What Gmail and Yahoo require from senders of roughly 5,000 messages a day: SPF and DKIM, DMARC, alignment, one-click unsubscribe and spam rates.
4 min read
Blog · Deliverability
Deliverability
SPF, DKIM, DMARC, BIMI, sender reputation and the inbox rules of Gmail, Yahoo and Microsoft: what decides whether your email arrives.
DeliverabilityVMC vs CMC: which BIMI certificate do you need?
Verified Mark Certificates need a registered trademark; Common Mark Certificates do not. Compare requirements, mailbox support and cost factors.
5 min read
DeliverabilityDisplay name spoofing: the attack DMARC does not stop
An attacker can send from any free mailbox with your CEO's name in the display field. Why authentication cannot stop it and what actually helps.
4 min read
DeliverabilityQuishing: QR code phishing and why filters miss it
QR codes hide malicious URLs inside images, out of reach of many link scanners. How quishing works, why it evades filters and what defenses help.
4 min read
DeliverabilityLocking down domains that never send email
Unused domains are easy spoofing targets. Publish a null MX, a deny-all SPF record and a reject DMARC policy so nobody can send mail as them.
4 min read
DeliverabilityLookalike domains: detecting cousin-domain phishing
Typo and homoglyph domains pass authentication for themselves. Monitor new registrations, register obvious variants and train people to check senders.
4 min read
- Deliverability
Why two SPF records are worse than none
Publishing a second SPF TXT record makes every check a permerror. How it happens, how to detect it with dig, and how to merge records safely.
4 min read
DeliverabilityThe case against noreply@ addresses
A noreply sender discards replies that signal engagement and frustrates customers. Route replies to a real inbox instead and set expectations clearly.
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
DeliverabilitySPF, DKIM and DMARC in plain English
Three DNS records decide whether your email arrives. What each one actually does, what to publish, and the four mistakes that cause most of the support tickets.
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
DeliverabilityBIMI readiness checklist: DMARC, SVG and certificates
Everything you need before a BIMI logo shows up: DMARC at enforcement, an SVG Tiny PS logo, an optional VMC or CMC, and a correctly formed DNS record.
5 min read