Regarding you comment about moderators being able to restore deleted sent emails. That is not correct. They have on a couple of occasions have been able to have some customers emails that were deleted restored but by no means all of them.
In most instances, when the moderators become involved all they can do is tell you which device triggered the deletion of the emails.
If this has happened to you before why are you are still using your sent folder as a storage folder. You would be far better and safer to set up a separate storage folder and keep your sent emails there.
You should also have a bag up regime where you back up all your email folders on a regular basis so that should something like this happen again you still have all your emails stored safely other than on the BT email servers.
As regards your problem, are you using a web browser or an email client/app to access your email account and if so which ones?
Are you using more than one device to access your emails and if so what devices and how do you access your email account on each one?
If you are using an email client/app is it set up as an IMAP account or a POP3 account?
Is it only your sent emails that are missing?
Have you checked all your other folders to see if the deleted emails are there?
