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Adding a default email address to all SFDC comments per account
How can I add a default email address to which all communications on cases will go to, for the account the case is opened for?
You might not need this as Salesforce automatically recognized customer response to Case automatically based on "Ref Id" in subject so that it is attached to right case. If you need any custom handling around this then you should be using "Ref Id" on Case only as there is not other way.