Auth with email name only

[Leonard Cooper]

Hi all,
Is it possible to authenticate using only an email name? We are running
vpopmail, all virtual domains.
The Client that requires this will becomming from a pre-determined IP
address range, so all users for this domain will be coming from that IP
range.
Or, alternativly, how can I set this domain to be the default, considering
all domains are virtual??

Thanks
Leonard