What this is
A provider-agnostic specification of observable outcomes. Each page states what the requirement is, why it matters, how to implement it, common mistakes, and how to verify the delivered result.
What this is not
It is not an ESP comparison, a template framework, legal advice, or a guarantee of inbox placement. Standards define the floor; mailbox providers and recipients still make their own decisions.
Open by design
Code is MIT licensed. Specification content is CC BY 4.0. Every topic cites its sources and the repository accepts corrections.