The advanced relay option makes possible to forward emails based on non-existent recipients and also to forward all the mail that comes to a domain. This last feature is also known as catchall (Figure 4.26).
Relay definitions:
Email/Domain
Email
: Insert a invalid recipient that doesn't have any LDAP account created and the desired internal domain;
Domain
: Choose for each domain you want to relay all the messages (catchall option);
Relay to
: Destination email. Can be an internal or external destination;