In Outlook, rules are composed of three parts: conditions, actions, and exceptions. A condition is the initial filter for the rule, which identifies properties of a message against which some action should be applied. If a message fits the condition, the action is executed unless there's a fitting exception configured that prevents the action. Within the condition, you can assert that the rule applies only to a specific domain.
In Outlook 2010, from the Home tab in the Outlook Ribbon, select Rules, Manage Rules and Alerts, then click the New Rule button to launch the Rules Wizard that Figure 1 shows. Similarly, in Outlook 2007 and Outlook 2003, you would select Tools, Rules and Alerts from the menu bar, then click the New Rule button in the Rules and Alerts dialog box.
Figure 1: The Rules Wizard in Outlook
In the section Start from a blank rule, choose the option Apply rule on messages I receive. Next, as Figure 2 shows, select the condition From people or public group. In Outlook 2007/2003, this shows as from people or distribution list.
Figure 2: Selecting conditions to apply in the Rules Wizard
In the lower half of the window, click the underlined people or public group to open the Rule Address window. Enter the SMTP domain you want this rule to apply to, as Figure 3 shows.
Figure 3: Entering the SMTP domain you want the rule to apply to
When you click OK, Outlook returns a Check Names window, claiming it doesn't recognize what was entered. Just click Cancel to apply the domain to the condition of this rule, as you can see in Figure 4.
Figure 4: The Rules Wizard with the condition applied
Over every version of Outlook, an annoying aspect of referencing email addresses or SMTP domains in Outlook rules is Outlook's insistence that the address (or in this case, a domain) reside in the Address Book (or Contacts). If the email address you're using within an Outlook rule doesn't reside in the Address Book, you can certainly add it before creating the rule to avoid the Check Names dialog box.
Click Next to configure the action and exceptions (if any) to complete the Outlook rule. Perhaps you want messages from a specific domain to be stored in a certain folder, or forwarded to another user for follow up. All of the Rule Actions are available. The rule will now apply to all inbound messages from an entire SMTP domain, no matter who the sender.