Person whose domain it is (hereafter abbreviated "H", that being her initial) reports that inbound email to her brother appears not to be getting to him. I check on the server that's supposed to do the rewriting -- everything looks fine. Test mail to the address to which it's forwarded, though, is sitting around and waiting to retry talking to the ISP's mail server. Try telnetting to port 25 from various servers (four servers on three networks); no dice, it just doesn't want to talk.
Scratch head a bit.
A quick search on Google Groups suggests that I'm not the only one having this issue, and furthermore suggests that Verizon (for it is they) may in fact be blackholing all of the UK as far as mail is concerned.
Suddenly remember I do have access to an account on a US-based machine. Log into that and try to connect to relay.verizon.net. Which works.
I advised my customer that she may want to suggest to her brother that he change ISP... not a whole lot I can do about that one.