[exim] SPA authentication and Vista

To authenticate users before they can send mail from my server, I have been
using SPA which has been working fine up to now.

One user has just switched to Vista and it has all stopped working -
authentication fails every time.

I guess Vista is encrypting the username/password differently to the way XP
is - does anyone have any experience on getting Exim to work with Vista when
using SPA (whilst keeping it working with XP)

Relevant config file section:

begin authenticators

spa:
driver = spa
public_name = NTLM
server_password =
${lookup{$auth1}lsearch{/usr/exim/exim-users}{$value}fail}


Regards,
Keith.