When you are looking at an email from a customer, you’ll see a list of all the other emails you’ve ever received from the exact same email address in the Correspondent section on the left side:
This lists other cases sent to or from the same email address. Clicking on the link before the @ sign (“janeclient” in this example) returns a full list of all email with this correspondent only. The correspondent field is editable, making it easier to manage related cases when an email thread involves more than one customer.
You can also search for other email from the same domain name by clicking on the link to the right of the @ sign in the email address (“test.fogcreek.com” in this example). This can help when several people corresponded with you from the same company.