I got fed up with getting no response from newton.digitalspace.net about the blacklists they're on, so I've now changed my LiveJournal address.
Mail now goes to a subdomain of my regular FastMail.FM address and is then forwarded on to Gmail by a rule in my Sieve script, so the effect is similar to how it was before, only now mail goes LiveJournal -> FastMail.FM -> Gmail rather than LiveJournal -> DigitalSpace -> (1) FastMail.FM + (2) Gmail.
So now I receive comment notification emails in my normal inbox again.
And a pox on misconfigured mail servers.
(Not that this should affect any of you. pne at livejournal dot com should still work, for example.)