BT Yahoo e-mail web mail client is deleting all emails from inbox, occasionally moving to trash without warning or even a user being present. Local clients such as Outlook work fine but BT Yahoo Mail doesn't. E-mails slowly go/delete with no pattern.
Make sure your e-mail clients are set to leave a copy on the server
There is no setting for this option on the web browser client that I use. (BT Yahoo in browser mail client) (1st party)
"Local clients such as Outlook work fine"
That is the one you need to alter, so that when it fetches messages, they are not deleted from the webmail account.