Your mailserver is being extremely picky about what it accepts. It's rejecting the user@host smtp envelope address tfl was sending mail from. The vast majority of mailservers ensure that host is valid, but not that mail for user@host is accepted.
The envelope from address is now accepted, but I'm not sure that's a good permanent solution, because that mail ends up going nowhere and is read by no one. I'm not in favor of that, because if mail isn't automatically processed or forwarded to a human, IMO it shouldn't be accepted at all.
The vbulletin notifications are entirely automated and there should not be a reason to ever need to reply to one, hence why the return address was not accepted.
“Australians have very bad habits – piracy happens to be one of them.” --News Corp CEO Robert Thomson, on Game of Thrones torrents
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