There is an anomaly with historic ntlworld email accounts in that no mail is deleted even when it is requested by a mail client that it is deleted. This is a fallback from when Virgin migrated accounts over from Google to their own servers. These messages are hidden from the client by the server & can only be deleted via webmail. The only way for a client to “see” them again is to mark them as unread.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.