Wildcard in custom users and groups

I have activate recipient validation and it prevent from sending mail to my fax server. The only way to make it work is to create a custom users with the exact adress which is the [fax_number]@domain.com. So my problem is that I cannot add all the fax number of the world so I need a way to use wildcard in custom users and groups.

Thank in advance and sorry for my English !