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Set up Email to Case to use the support2 support2@ email address.
In your email management system (eg Exchange or Google Apps) set up support@ to forward to the Salesforce created Email Services Address for support2@.
So incoming email goes from support@ directly to support2@ to the SF Email Services Address set up for support2@ and then into Salesforce.
Now set up an organization wide email address as support@
Now, set up your Case Auto Response rules to use support@
Ensure the Email Template for the Auto Response rules have the Case Thread ID in the Subject and Body.
Now, the missing link that I missed - in your Case Feeds settings enter the support@ address as the sepecific email address to send from.
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