http://members.elysium.pl/brush/qmail-smtpd-auth/ might be what you're
looking for. Just replace /bin/checkpassword with /home/vpopmail/bin/vchkpw
and it should work with vpopmail.
> Hello All, I've been lurking on this list a while (trying to learn about
> the server I run), and I have a problem.
>
> I need some way of letting users from dynamic IP's across the internet
> use my SMTP server, but I just got used as an open relay. SMTP-after-POP
> won't work for me (windows users, they won't accept that), so I was
> thinking about Authenticated SMTP, which is an option in Outlook/Outlook
> Express which they all use. Can anyone give me some pointers on setting
> this up so it uses my Vpopmail usernames and passwords the same was POP3
> does? I'm really rather new to the unix scene.
>
> Thank you,
>
> David Crawshaw