Migrate from old sendmail to cwp postfix

Hello,   

Thanks for allowing me in this forum. I have an old sendmail server which i want to shift in cwp latest one. However i am unable to find how can i enable login from mail client to cwp server with only id portion (i.e. without domain part). As all of our users (approx 500) all already configured their mail client (ms outlook, mozila firefox) with id part only we need this to transfer. Any help will be appreciated.

Thanks

Towhidul