roaming user with qmail

Hi i have configured qmail + Vpopmail all the mails are fetched from
pop accounts on the domain mydomain.com & then filtered to my local
qmail mailboxes.
everything else is working ok. Ony one thing i want to configure is

whenever a local user is travelling. He wants his mail to be accesed
by webmail on my internet domain & also mails from local user should
come to the domain
& not on local mailbox

any idea how i can accive this

Thanks

Wyseboy