[otrs] customer username and from:

I'm new on OTRS. I'm running OTRS 3.0.6. How tell to postmaster to associate a received email to a customer user name (if the user name is not a email) ? Of curse the customer I used to test have the "Email" field filled right, and the email field "From:" (received by postmaster) has the exactly email of the Customer. Sincerely, Rogerio.