By default, content filtering rules for inbound SMTP messages apply to messages that have at least one recipient who has a mailbox in the Exchange organization. Rules for outbound SMTP messages apply to messages that have at least one recipient that does not have a mailbox in the Exchange organization.
You can modify these settings by specifying the domains that your organization considers local. By adding a domain to the domain list, emails with recipients for that domain are considered local, even if they do not have local mailboxes.
A single message can be considered both inbound and outbound. In this case, both inbound and outbound rules are applied to the message.
To specify inbound SMTP domains
In the console on the primary navigation bar, click Admin.
In the sidebar under Views, click System Settings.
In the content area, under System Settings, check Enable list of internal domains.
In the List of internal domains box, type the domain or domains that define which email message domains are inbound.
Type only one domain per line.
On the toolbar, click Deploy changes to apply your changes.