Authentication when opening web pages from Outlook

[Andrew J]

I have an intranet asp application that sends emails that contain a link to
an intranet page. I have a case where one user is forced to login to the
windows domain when he clicks on the link, even though he is within the
firewall & his Outlook security settings specify automatic login with the
current name & password. This doesn't happen with any other users (unless
they go through the firewall). The site is also recorded in the trusted
sites section.

Any ideas?