Hello Tollyfromslawit
I have experienced exactly the same problem, all my saved Folders and the emails in them would not load up after BT ungraded its email format. Sent and Draft Messages Folders also empty. I am using an Apple Mac Pro purchased in 2017 running Big Sur operating system and the latest Safari browser.
I spoke to BT Tech Support initially and on Tuesday this last week I had a long telephone conversation with their Tech Support Team in Newcastle.
We tried various browsers, Firefox, Chrome etc with the same result, the Folders would not load. I sent them a screen shot showing the landing page where the Folders use to appear. Having said that every now and again the Folders did appear after a time but as soon as you had to reload the BT email service, they disappeared again.
The best that BT could come up with was it must be my out of date operating system and they suggested I contact Apple support for advice. However, I don't think this is correct. I also have a windows laptop running the latest version of the Edge browser and my Folders do not load up when I use this to access the BT mail service either so I think the problem is their end not mine.
I am veery reluctant to spend £2000 plus to purchase a current Mac Pro laptop running the latest operating system only to find it doesn't cure the problem.
Currently I am now getting a "The Service is experiencing problems, try again later" error message when I attempt to use BT email.
As suggested elsewhere I am using Thunderbird email client and Apple Mail to access my emails and all my Folders are there but I still have a few problems with both of these such as my email account passwords not being recognised although I know they are correct.
I am expecting call back from BT next Tuesday but I don't think they will be able to determine what has gone wrong following the email upgrade. It is obvious from the number of complaints made to the BT Community there are some significant bugs to be sorted out.
I hope this helps
Regards
Peter Kemp
