When a Case is created through Email-to-Case, the Case ownership is not changed as per the Case assignment rule. The assignment rule will not fire because the 'Case Owner' field on 'Routing Address' under Email-to-Case has User or Queue selected and this will override the Default Case Owner field that's specified on the Support Settings page and in the assignment rule.
If the Case Owner field on Routing Address is left blank, then the assignment rule will fire any time the Case is created through Email-to-Case.
Configure Routing Addresses for Email-to-Case and On-Demand Email-to-Case
Routing Address Settings for Email-to-Case and On-Demand Email-to-Case
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