Google Workspace's default DKIM signing domain for a mailbox domain
that has no DKIM key of its own: dots replaced by dashes, then
.<digits>.gappssmtp.com.
Parameters
identityDomain: string
The signing domain from a dkim= clause.
senderDomain: string
The domain of the From address.
Returns boolean
Whether identityDomain is senderDomain's gappssmtp signing
domain (case-insensitive); false for a sender domain with characters
outside [a-z0-9.-].
Google Workspace's default DKIM signing domain for a mailbox domain that has no DKIM key of its own: dots replaced by dashes, then
.<digits>.gappssmtp.com.