Thank you for the information, Eddie.
I have asked the ntl helpline about this. They said that the @dtn.ntl.com addresses are no longer in use by them, and that some third party must have configured their own mail server to use them. When I asked how could mail addressed to
myname@dtn.ntl.com be successfully delivered to me when that was not a valid address for me, I was told that the server must be employing 'carbon copying' whereby anything addressed to the @dtn.ntl.com address was redirected to the corresponding @ntlworld.com address with the headings forged so as to hide the fact. Their advice was simply to treat all such mail as spam.
I cannot believe that some spammer has gone to these lengths just to target me. Is nobody else getting these 'dtn' bounces?
A whois search on dtn.ntl.com drew a blank. Call me paranoid, but I cannot help feeling that there is more to this than just another spammer's ploy.