While configuring ELA, you have the option to set the default attributes of the Class-of-Service and Community ID. The default Enhanced-Lists can receive from any community and send messages to any community. This list is different from the Shadow Mailbox, where the shadow mailbox is analogous to a postmaster. Subscribers cannot reply to a shadow mailbox list.
To set up the ELA default community ID, you need to administer a CMM Classes-of-Service with the recommended values for ELA.