I have the same issue. Today I have found out that I only have about 30 emails.... thousands have disappeared.
I have not done anything differently.
The strange thing is that some emails that were in files are still there but I don't tend to use that facility so all my house sale documents, rental details etc have gone. But the sent file is still there going way back. Inbox has almost all gone, only the last few days available.
This is very upsetting, I have lots of emails that I need.... I require access to documanets that were attached to them etc.
Has there been an issue at BT?
How do you access your emails, is it with a web browser (Web Mail) or using an email client app or a combination of both?
How many devices do you use to check your email and what are they?
Have you checked all the devices to see if the emails are in the their trash/junk folders?
If you use an email client/app have you checked by logging onto webmail to see if they are still on the BT server?
If they are not on the BT server it means that they have been deleted and can not be recovered. All BT may be able to do is to advise you what device triggered the deletions.
I use Google Chrome on my pc and also have the mail on my iphone and ipad.
It was only when I was looking for an email from a few weeks ago to reply to again that I realised they had all gone. I have inbox emails only from last Monday onwards (6th Feb).
Can there be security issues on iphone? Could it have been hacked or something? I had been away the week prior to that and connected briefly to airport wifi, plus used hotel wifi.
I have just been through security advice online and changed password, security question, checked no activity been added (forwarding emails/new contacts etc).
There have been similar reports on the forum and one of the common things is that all the users used iPhones or iPads to access their email and when BT checked to see which device triggered the deletion it was either their iPhone or iPad.
If you have checked and the emails are not on the BT servers when you use webmail it means that they are gone and can not be recovered by BT.
Not much use now but in future do not use the email inbox as a storage folder. If you have emails that you want to keep you should set up a new folder and store them there or better still download the emails to your PC and store them in a folder there. You could easily do this by setting up an email client on your PC and set it as a POP3 account instead of an IMAP account which you would appear to be using now on your Apple devices.
You should also include the email folders in your backup regime, assuming you have one.
It is possible that your phone had been hacked but obviously it is not possible to say for certain that is what happened or indeed if it ever happened.
Normally when an email account is hacked the hackers usually harvest the hacked accounts contacts and at some point send phishing emails purporting to be from the hacked account holder to the contacts in an attempt to get something from them or get them to click a link or download malware onto their device so it may be worth informing your contacts to be aware of any such type of email purporting to be from you.
I've had the same problem last night, I've started a thread about it, and funnily enough I checked my emails on my ipad last night, I don't normally use an apple device usually a pc with chrome.