Log onto your email account then click on settings found by clicking on your username. Go to Mail then Block senders List and scroll through until you find the ones you want.
Thanks DavidM, @gg30340 and @licquorice, it is actually the blocked senders and domains that may have been blocked on the BT web site that I am looking for. I am not sure whether BT have blocked emails from people or whether they have added my entire address list to the blocked listing by mistake.
Thanks for you help
The advice you were given was in relation to the BTMail system. I am not sure what you mean about " blocked senders and domains that may have been blocked on the BT web site"
If you explain exactly what you mean perhaps somebody may be able to assist.