Koltrix
Deliverability
Why DMARC breaks mailing lists, and how ARC helps
Forwarders and mailing lists modify messages and break SPF and DKIM. Understand the failure, From rewriting, and what ARC chains do and do not fix.
4 min read
Blog · Deliverability · page 4 of 5
Deliverability
SPF, DKIM, DMARC, BIMI, sender reputation and the inbox rules of Gmail, Yahoo and Microsoft: what decides whether your email arrives.
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
DeliverabilityWhy transactional email lands in Gmail Promotions
Receipts and alerts in the Promotions tab are usually a content and stream problem. The signals Gmail likely uses and changes that tend to help.
4 min read
DeliverabilityDetecting a compromised account sending spam as you
A hijacked mailbox or leaked SMTP credential can burn your domain reputation in hours. The signals to watch and the containment steps to rehearse.
4 min read
DeliverabilityShared vs dedicated sending IPs: a decision guide
Dedicated IPs give you control and responsibility; shared pools give you a neighbor's reputation. Volume thresholds and questions to decide well.
4 min read
DeliverabilityIP warm-up: a realistic schedule and what to watch daily
A practical IP warm-up plan that ramps volume over weeks, prioritizes engaged recipients and backs off on deferrals, with daily signals to check.
4 min read
DeliverabilityGreylisting 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
DeliverabilityHow email spoofing works and which records stop it
SMTP lets anyone claim any From address. Follow a spoofed message through a receiver and see exactly where SPF, DKIM and DMARC stop it, and where not.
5 min read
DeliverabilityTransactional vs marketing mail under bulk sender rules
Which bulk sender rules apply to receipts and password resets, which apply only to marketing, and why mixing both streams on one domain is risky.
4 min read
DeliverabilitySpamhaus listings explained: SBL, XBL, PBL, CSS, DBL
Spamhaus runs several blocklists with different causes and fixes. What each one means for a sender, how to check a listing and where to start.
5 min read
- Deliverability
DKIM replay attacks: how signed mail gets abused
A single legitimately signed message can be resent to millions. How DKIM replay works, why it hurts your domain, and the mitigations available today.
4 min read
- Deliverability
Relaxed vs strict DMARC alignment: choosing adkim and aspf
Relaxed alignment accepts subdomains, strict demands an exact match. See how each mode evaluates real headers and when strict is worth the risk.
4 min read