I’ve had multiple reports this morning of BTInternet customers seeing mail just disappear from their accounts.
I’ve looked into this and narrowed it down to users using Outlook via IMAP.
The symptoms are that when a new message is read, one or more existing messages below it in the inbox disappear within a few minutes.
There is a thread on the BT forum about this issue that I am following to keep up with developments, which you can see here: https://community.bt.com/t5/Email/Emails-disappearing-from-inbox/td-p/1977636 and there are similar posts relating to Sky elsewhere (and apparently AT&T who also use Yahoo email).
According to forum postings online, Yahoo have given a predicted date of November 7th/8th for completion of the fix of the disappearing mail problem.
In the meantime, do not use Outlook for accessing your BT email accounts. You can set up a forwarding rule in the BT webmail interface to forward the mail to a non-bt email address, and then access that without fear of losing mail, or simply use the BT web interface at BT.com for mail until the problem is fixed.
This is a huge mess-up by Yahoo, and all BT customers who had not been migrated from Yahoo to the new BT Mail system when BT silently abandoned the migration in 2015 are being affected by it (if they use Outlook as their mail client).
If you know anyone else who is affected, please spread the word. Also, I think it would be useful for anyone affected to place a formal complaint with BT about this problem.
The problems appear to have been fixed now, and all email appears to have been restored.