Problems accessing Outlook 2007 through mail server

Hello,
Anybody please assist with our Outlook 2007 problem!
When employees are opening Outlook they are being requested to validate to our mail server…mail.school.worcs.sch.uk
If they visit their user/pass they are able to load outlook and continue as usual.
OR
If they login to OWA webmail.school.worcs.sch.uk they are able to load outlook.
I don’t know what’s going on!!
I have attached the internet requests in below. It happens when they insert their credentials…
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 –
Please help. Thanks a lot!
