Sorry for replying you late, I waited for you reply and went offline for the day after 3 hours. Here are the steps to setup Outlook 2007, please try them out and let me know the result.
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Open Outlook 2007
Click on the Tools menu then select Account Settings from the drop-down menu.
Select the E-mail tab and click New.
If you are asked to Choose Email Service then please continue with the pre-selected option and click Next
In the Your Name
box enter the name you want people to see when they get your emails.
Enter your full BT Yahoo! email address and your password. Click Next
Outlook will now check your settings work. When it's done, click Finish
Select the "btinternet.com" email account and click on Change
Click on the More Settings
button in the bottom right-hand corner
Select the Advanced
tab and tick the box next to "Leave a copy of messages on the server". This means your emails will be on the BT servers so you can access them from other computers or internet devices. Click OK
Click the Next
to complete the set-up.