With this Swantide Workflow, when you have a Case created and if the Case is not related to an Account, Contact or Lead, you can relate and update the Case to one of these records based on the Email Address that is populated under the field "Web Email" (API Name - Supplied Email).

In the event that a Case comes through and Web Email is populated, the flow will start looking to match an unconverted Lead with the same email address. If a Lead is found, then the Case will be updated with the two custom fields called "Lead Name" and "Lead Email"

If a Lead wasn't found, the Flow will then continue to see if a Contact has the same Email Address under the field "Web Email". If a match is found, the Case will be updated to the standard field "Contact Name."
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Furthermore, if the Contact belong to an Account, the Case will also be updated with the Account information under the standard field "Account Name"

To deploy this workflow from the app, click here.