I'm sorry my explanation was not clear enough. For example, you receive an email. A new case is registered based upon this email. Once you open the case, you can find this first email received in the "Processing tab". The email address the request was sent to can be seen there.
There is no information stored in the case table regarding the email it was registered from. Usually, the category field defines the mailbox this case came from.
Hopefully, it's clear now, but please get back if you have any further questions.