Final answer:
In email delivery content, a mirror link and an unsubscribe link should always be included for legal compliance and to provide recipients with viewing options.
Step-by-step explanation:
The element that should always be included in the content of an email delivery is A. A mirror link and unsubscribe link. This is vital for email marketing campaigns to comply with laws such as the CAN-SPAM Act, which mandates the inclusion of an unsubscribe mechanism in all commercial messages.
Including an unsubscribe link in your emails ensures that recipients can easily opt out of receiving future emails if they wish. It is also best practice to include a mirror link, which is a direct link to a web-hosted version of the email content, allowing recipients to view the message in their web browser in case their email client fails to display the content correctly.