[otrs] how to handle customers without email adress ?

Hi, OTRS doesn't allow to create customers without an email adress. I guess because it's used as a unique id. Does anybody know the supposed way to handle customers that only have a telephone number but no email adress ? Or can someone recommend a best practice ? Thanks in advance. Alexander -- radprax Gesellschaft fuer medizinische Versorgungszentren mbH, Bergstr. 7 - 9, 42105 Wuppertal, Fon: +49 202 2489 1110, Fax: +49 202 2489 94 1119 Geschaeftsfuehrer: Andreas Martin, Dr. med. Heiner Steffens, Dr. med. Renate Tewaag Amtsgericht Wuppertal: HRB 19359, St.-Nr.: 132/5889/0264, DE 814559152 Web: http://www.radprax.de