When emails arrive they are processed automatically and stored in the relevant mail boxes.
If a customer sends you a new email then the status will be NEW but if you send the customer a reply, or send the customer a new email the status will be CUSTOMER. If the customer then replies to your email the status changes automatically to OPEN. This makes it very easy to track which emails you need to be dealing with, and which emails need chasing up as you are waiting for a reply. Combine this with a basic categorisation of mail boxes then you start to have a powerful communications management facility.
When a new email arrives from an unknown contact you can choose to mark the email as spam, add the contact to an existing company or create a new company entry. When subsequent emails arrive from that email address they will be automatically tagged with the appropriate details. So you can then quickly start to filter all the communications that you have with a particular customer or supplier.