The advanced relay option makes possible to forward emails based on non-existent recipients and also to forward all the mail that come to a domain. This last feature is also known as catchall (Figure 4.23).
Relay definitions:
Email/Domain
Email: Insert a invalid recipient that don't have any LDAP account created and the internal domain;
Domain: Choose for each domain you want to relay all the messages;
Relay to: Destination email. Can be an internal or a external one;