HowTo: DMARC

Organizational Domain:
The domain that was registered with a domain name registrar. In the absence of more accurate methods, heuristics are used to determine this, since it is not always the case that the registered domain name is simply a top-level DNS domain plus one component (e.g., "example.com", where "com" is a top-level domain). The Organizational Domain is determined by applying the algorithm found in Section 3.2.
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The Organizational Domain is determined using the following algorithm:

1. Acquire a "public suffix" list, i.e., a list of DNS domain names reserved for registrations. Some country Top-Level Domains (TLDs) make specific registration requirements, e.g., the United Kingdom places company registrations under ".co.uk"; other TLDs such as ".com" appear in the IANA registry of top-level DNS domains. A public suffix list is the union of all of these. Appendix A.6.1 contains some discussion about obtaining a public suffix list.
2. Break the subject DNS domain name into a set of "n" ordered labels. Number these labels from right to left; e.g., for "example.com", "com" would be label 1 and "example" would be label 2.
3. Search the public suffix list for the name that matches the largest number of labels found in the subject DNS domain. Let that number be "x".
4. Construct a new DNS domain name using the name that matched from the public suffix list and prefixing to it the "x+1"th label from the subject domain. This new name is the Organizational Domain.

Thus, since "com" is an IANA-registered TLD, a subject domain of "a.b.c.d.example.com" would have an Organizational Domain of "example.com".

The process of determining a suffix is currently a heuristic one. No list is guaranteed to be accurate or current.