7. RFC4871 Section 2.10 Agent or User Identifier (AUID)
A single, opaque value that identifies the agent or user on behalf
of whom the SDID has taken responsibility.
A single domain name that identifies the agent or user on behalf
of whom the SDID has taken responsibility. For DKIM
processing, the name has only basic domain name semantics; any
possible owner-specific semantics is outside the scope of DKIM.
Are we changing the format of the AUID from an email address where the
lhs is optional to a domain name?
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