The signing domain from a dkim= clause.
The domain of the From address.
True when the two are equal (case-insensitive), or when the
sender domain has exactly two labels and identityDomain is its
gappssmtp alias (isGappssmtpAlias); a sender domain of three
or more labels needs the exact match (the alias collides there).
Whether a dkim identity domain authenticates
senderDomain.