My blueyonder email address shows a user name which I have no knowledge of.
Thanks for the update.
I did not have a Blueyonder email account - my "pre Virgin Media" email address was an Ntlworld address. However, I do know that the old Blueyonder accounts had the option of creating an alias address for each email account.
Alias addresses were not separate, secondary email accounts the were merely an alternative address for the main Blueyonder account. Virgin Media still support those that exist but as far as I know VM soon stopped new Blueyonder aliases from being created.
If this mystery other user refers to a Blueyonder.co.uk email address then I suspect that it is merely an alias to your main email account. You could try sending a test email to it and see if the test email arrives in your main Blueyonder account. If it does then that proves it's merely an alias address for your main Blueyonder account.
Based on lots of other post on this forum over the years these alias accounts do often seem to get forgotten about and cause confusion. It could well be an alias was set up years ago and possibly never used.
If it is, than just to let you know, as far as I am aware there is no way for either you or VM to delete it. It's not doing any harm though so it's not a security issue.
Coenoby