[otrs] Mapping a sender email with a real customer

Hello, I'm considering using OTRS for our support system which is linked to a database containing our customers. I have setup OTRS 1.2.4 to use this as the customer_user backend. After I clicked on the "Customer" link of the "Zoom Ticket", I have a screen with two fields and a "Search Customer" link. The first one is filled with the ticket sender email, and the second as well. Problem is : I can't find what the "Search Customer" button does... I would like it to find a real customer (ie a proper customer id, if I understood well) from that email. Thanks for your time, Sylvain Thomas Illicom