Email to Case (standard) functionality uses the default case owner defined in Setup > Customize > Cases > Support Settings to assign cases to a default user or queue when cases do not meet the specified criteria for assignment. AppExchange apps such as Email to Case Premium are not able to use this default case owner functionality.
Instead, simply create an assignment rule entry that falls last in the order of evaluation and which specifies no criteria. This rule entry will catch any cases that did not match the criteria in the previous assignment rule entries and assign them to the specified user or queue.
Navigate to Setup > App Setup > Customize > Cases > Assignment Rules
Choose the appropriate Assignment Rule.
Note: If you do not already have an Assignment Rule created, click the New button to create one. Choose a rule name and click Save. Click the checkbox next to Set this as the active assignment rule. Click the Save button.
In the Rule Entries section, click the New button.
In the Sort Order field in Step 1, type in the number that would place this entry last in the Assignment Rule. For example, if you have two previous entries for the Assignment Rule, add this new entry as 3.
In Step 2, leave the Run this rule if the following dropdown as criteria are met. Leave the Field, Operator, and Value fields blank.
In Step 3, choose the user or queue that should receive the cases when the previous Assignment Rule entry conditions are not met.
In Step 4, optionally choose a predefined case team.
Click the Save button to save the new Assignment Rule entry. Also verify that "Use Active Assignment Rule" is selected on the E2CP Inbound Configuration page.