1. Поиск SMTP коннекторов для удаленного домена.
2. Если коннектор найден, то выбирается способ доставки Smart host или DNS в зависимости от настроек коннектора.
3. Если не найден, то настройки о способе доставки Smart host или DNS берутся по умолчанию на SMTP сервере.

Опимально использовать коннекторов SMTP, т.к. у Вас больше развязаны руки по управлению маршрутизацией.

могут быть такие варианты:
- передает смарт хосту если прописан в …Protocols SMTP Delivery Advanced
- передает SMTP коннектору если есть адресное пространство или оно *

http://www.redline-software.com/rus/support/articles/msexchange/2003/exchange-server-2003-mailflow-part1.php

Это читали? http://technet.microsoft.com/en-us/library/aa997477.aspx

Sending Internet Mail
Assuming there is a constant Internet connection, Exchange sends Internet mail by the following methods:
• It uses DNS directly to contact the remote mail server.
• It routes mail through a smart host that assumes responsibility for DNS name resolution and mail delivery.
Before each of these methods is described in detail, you should have a general understanding of how outbound mail flows in an Exchange organization.
Outbound Internet mail flows through an Exchange Server 2003 server in the following manner:
1. An internal user sends a message to a recipient in a remote domain.
2. To determine if the recipient is local or remote, the SMTP virtual server on the sender’s Exchange server uses internal transport functions to query the global catalog server for the recipient address. If the recipient address on the message is not in a recipient policy, it is not stored in Active Directory; therefore, Exchange determines that the message is destined for a remote domain.
3. If necessary, the Exchange server delivers the message to the appropriate SMTP virtual server.
4. The SMTP virtual server uses its IIS metabase information to determine the method for delivering a message to a remote domain.
5. The SMTP virtual server on the Exchange server then performs one of two actions:
• Uses DNS to look up the IP address for the target domain, and then attempts to deliver the message.
• Forwards the message to a smart host that assumes responsibility for the DNS resolution and delivery of the message.