SpamAssassin Rule: DKIM_ADSP_CUSTOM_HIGH
Standard description: No valid author signature, adsp_override is CUSTOM_HIGH
The presence of this test indicates that DKIM support is enabled on the server.
The mail does not contain a valid DKIM signature and the domain of the sending address has been matched by an override setting (adsp_override).
For frequently seen domains that either do not publish an ADSP (Author Domain Signing Practices) records, or where a stronger (or weaker) policy is desired, the values can be specified in the configuration, saving the need for a DNS lookup.
The current default custom_med overrides in 60_adsp_override_dkim.cf include entries for youtube.com.
One reason for not seeing a signature on a mail from an domain which routinely signs them is that the signature header may have been stripped out, for example as part of a mailing-list expander.
The default scores for this rule can be found in the online list of tests.