You need to ensure that the Edge Transport server can send and receive e-mail messages from the Internet for the organization

Using an Edge Transport server, you can provide anti-spam, antivirus, and transport rules processing for your Exchange organization. You can deploy and configure an Edge Transport server to act as a smart host in the perimeter network of an existing Microsoft Exchange Server 2003 organization before you start upgrading your existing Exchange 2003 servers to Exchange Server 2010. Although it’s a better practice to introduce Exchange 2010 into your Exchange 2003 organization and use Edge subscriptions, you may want to start using Edge servers before you start your upgrade. This deployment option may be desirable in the following scenarios:
– You want to start utilizing the benefits of Edge protection before beginning the upgrade of your internal Exchange organization.
– You have multiple sites that won’t be upgraded all at the same time, and you want the servers in these sites to send mail directly to the perimeter before they’re upgraded.
– You have an Exchange 2010 deployment and your organization merges with or acquires another organization that has Exchange 2003 deployed, and you want to centralize mail flow through your Edge servers.

Note:
In the scenario described in this topic, no computers running Microsoft Exchange Server 2010 have been deployed in the Exchange organization. This limits the available features on the Edge Transport server because you can’t use any of the features that rely on Edge Subscription. The features that rely on Edge Subscription are recipient lookup and safelist aggregation. If you want to create an Edge Subscription, you must deploy at least one Exchange 2010 Hub Transport server in the Exchange organization and configure the organization for coexistence. For more information, see Upgrade from Exchange 2003 Transport.