You're right that the email address no longer exists.
A system deleted mailbox does not automatically remove the entry from the list of Mailboxes in your customer MyAccount. But that means you cannot delete the entry from your MyAccount. The reason is that the delete process needs to find an active mailbox to complete its process. Therefore as the automatic processes are not set up for this scenario a manual intervention is required.
Either TalkTalk Support will remove that entry or seek approval from the Data Protection team to delete the entry or ask you to Contact the Data Protection team to have your deleted mailbox address removed from your MyAccount. You can initiate that approach yourself via Customer Services.
But the next part of your request is to create an additional mailbox as the TalkTalk policy says you can have up to 5 mailboxes. The reality is that TalkTalk have withdrawn the ability to add and create new mailboxes. We haven't been told why or when the creation of mailboxes might be returned.
