You need to create mail routes if this mail server is to relay mail to another machine. Mail routes are used to forward mail for a domain to the machine serving as mail store for that domain.
Each mail route has a domain name and a destination. A mail route tells the server to relay mail for a particular domain to a particular machine. You must create a mail route for every domain this server is to relay mail for.
In our example setup, the machine smtp.corp.guardiandigital.com should relay all mail for the domain corp.guardiandigital.com (Eg: mail to sales@corp.guardiandigital.com) to the machine mailbox.corp.guardiandigital.com. So, a mail route should be created in smtp.corp.guardiandigital.com that forwards mail for the domain corp.guardiandig-ital.com to the machine mailbox.corp.guardiandigital.com.