[Exim] Verifiying anyuser@validdomain

Hi,

Im not sure what to edit, I would like to verifiy <anyuser>@domain. This is
definiately not for an open relay. It is for users to send from any
user@their-domain. Im not sure what to edit in the configuration. This is
the error I receive now.

2003-01-21 12:05:49 H=(spitfire) [64.235.97.250] (hidden-user) sender verify
fail for <blah-blah-blah@???>:
Unrouteable address

(I want to route anyuser@domain)

Any help would be appreciated.

Thanks