Very slowly.
You can not delete them all at once.
Click on the box to the left of the first email you want to delete and assuming you want to delete all the emails that are below that you can either press on your keyboard "shift" key and "PgDn" key repeatedly and it will mark all the other emails with a tick or use the "shift" key and the down arrow. This will also mark all the boxes with a tick or tick the first box you want to delete and holding the shift key move to the last email you want to delete and tick its box and release the shift key. All the emails in between should now be ticked.
If you don't all the emails that have been marked with a tick deleted you need to go down the list and untick the ones you want to keep.
Once you have done that you then click on the "Delete" button in the menu bar which should delete them on mass however if it does not work, once they have all been ticked, you should try to drag and drop them into the trash folder.
If the above is to big a task you could install an email client/app, setting it up as an IMAP account and you should be able to select and delete all the contents of the inbox and this should be synchronised with the BT servers .
