Do you check your emails on more than one device such as on a mobile phone or tablet?
If you do have more than one device, do you move your emails from the inbox on one of your devices but not on the laptop?
I have a tablet and a phone the messages stay on my phone but not on my table or laptop. If I delete on any device it automatically syncs with the others
If you have set your phone and tablet email app up as an IMAP account they should both synchronise and replicate what is showing on webmail, ie when you log onto your BTMail email account using a browser.
This should mean that if the inbox is being emptied on one it should be exactly the same on the other.
How do you access you email account on your laptop. Is it with a browser or using an email client/app?