RFC 6376, "DomainKeys Identified Mail (DKIM) Signatures", September 2011Source of RFC: dkim (sec)
Errata ID: 5260
Reported By: Ale
Date Reported: 2018-02-08
Rejected by: Barry Leiba
Date Rejected: 2019-04-30
Section 3.5 says:
It should say:
DKIM-Signature = "DKIM-Signature:" tag-list
A formal definition is needed to make it explicit that this header field name is case insensitive, like all the other header field names.
Header field names are case-insensitive, in general; there is neither need nor desire to repeat that in the DKIM spec.