VM's mail exchangers definitelty respond correctly in that regard.
Just checked with my old blueyonder.co.uk address which I know for a fact has been deleted.
And just to clarify all four Virgin Media email domains use the same mail exchangers, so if the mailbox has been deleted properly it should definitely be throwing a 550 5.1.1 MXIN501 <email address> unknown error.
The Forum Team should be along presently so they can take a look and get the info they need to pass to the email team to fully close down the mailbox.
Tim