What must an organization do to set up dual delivery of mail using a subdomain?

To successfully set up dual delivery of mail using a subdomain, it is essential to point the MX (Mail Exchange) record of the subdomain to the appropriate mail server. In this case, directing the MX record for the specified subdomain to aspmx.l.google.com indicates that the mail for that subdomain will be handled by Google's mail servers.

This configuration allows incoming emails for addresses under the subdomain to be correctly routed to Google Workspace's infrastructure, enabling users to receive emails in their Google Workspace mailboxes. This setup is integral for ensuring that dual delivery functions correctly, as it establishes where the mail should be directed for processing and delivery.

The other options do not effectively address the requirement of pointing the MX record to Google's mail servers, which is crucial for setup. For instance, simply pointing the MX record of the subdomain to the primary mail domain doesn’t provide the necessary direction for handling emails specifically for the subdomain. Additionally, while configuring SPF records and verifying ownership may be necessary steps in a broader mail setup or security context, they do not specifically ensure the routing of mail through the designated mail servers, which is the key requirement for establishing dual delivery through a subdomain.