Logging in needed before loading MS Outlook 2007

Hello everyone,
Can I ask for help with our annoying MS Outlook 2007 problem?
Whenever our staff try to open Outlook, they are asked to authenticate our mail server, mail.school.worcs.sch.uk. They have to either type in their username and password or login first to the OWA, webmail.school.worcs.sch.uk, before they can load Outlook.
I don't understand what's going on.
I capture some internet requests with PuTTY whenever they type in their credentials. I am wondering if the Outlook in school is accessing Webmail on HTTPS?
1299661532.801 0 10.50.4.15 TCP_DENIED/407 1308 CONNECT webmail.school.worcs.sch.uk:443 – NONE/- text/html
1299661532.807 3 10.50.4.15 TCP_DENIED/407 1620 CONNECT webmail.school.worcs.sch.uk:443 – NONE/- text/html
1299661533.021 208 10.50.4.15 TCP_MISS/000 12945 CONNECT webmail.school.worcs.sch.uk:443 test.teacher SOURCEHASH_PARENT/Staff –
Thank you in advance!
