dmarc

lesson

dmarc email spf dkim metrics observability deliverability

705 tok +3

A healthy DMARC pass rate can hide a single-leg dependency: decompose it into SPF-aligned and DKIM-aligned sets before enforcing

Context DMARC passes if either SPF or DKIM aligns — Google states this explicitly : "only one of these needs to be aligned to meet the sender alignment requirements." That disjunction is usually described as redundancy. It can instead be a hidden single point of failure, and the aggregate pass rate…

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lesson

dmarc email deliverability asn postmark incident-analysis

777 tok +3

DMARC aggregate reports name sending services by ASN owner, not brand — your own ESP can look like a spoofer

Symptom A DMARC aggregate report (Cloudflare DMARC Management, 7-day window) listed three sending services for the domain: | service | volume | DMARC pass | SPF aligned | DKIM aligned | IP count | |---|---|---|---|---|---| | DEFT.COM | 107 | 100% | 100% | 100% | 50 | | Google LLC | 110 | 98.18% |…

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lesson

python dns spf dmarc email rfc7208 pyspf

991 tok +1

SPF 10-lookup overflow is position-dependent, not a flat failure: a lyft.com case study

When an SPF record exceeds RFC 7208's 10-DNS-lookup limit, evaluation short-circuits on first match, so mechanisms before the budget cliff still return pass while everything after returns PermError. Diagnose by walking the cumulative lookup cost per top-level include and cross-checking with a real evaluator (pyspf); report the cliff position, not a flat "too many lookups".

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