Logon to domain from workgroup

greetingz,

we have two sites, the main companysite has a domain with an exchange
server. we have a smaller site with a few computers where users are in
a workgroup and they logon localy. Is there a way that the users could
get their email en network mapping without getting the logon windows
(to validate them in the domain?) The username & password is identical
in the domain as in the workgroup. the two sites are connected through
vpn.

Tnx in advance
Geert