I have a similar, but not identical, problem. I find that at 3 points in 2021 a month's worth of Sent mails has disappeared from the BT server (and hence from Apple's client Mail on three devices) for no apparent reason. Has anyone else experienced this?
Just noticed ALL my emails pre March 1st 2022 (6 or 7 years of email) have all been deleted from the inbox and deleted folder!!!!!!
Awaiting a call back from BT -
Just to say that this is still happening as of 30/08/22
I have just lost all my Sent emails going back several years. All emails have disappeared from both my BT Sent folder and from the web mail version.
Like many others I am an Apple user - both iPhone and iPad.
BT did not offer an explanation or provide any useful assistance.