This may not be the correct place for this question, if so please move it to the right forum.
I am unable to find information on MX records with regard to redundant email addresses.
If I want to use the site email as the primary and another email (not on that site's mail server) as a secondary and third as a backup how do I go about this? Does the email domain matter in this case?
I don't quite understand the question. Do you want to forward emails to another email address if the primary mail host is unavailable. Usually if the primary mail host is unavailble, the email gets sent to the other MX record hosts, and those hosts just keep trying to send the email to the primary mail host.