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.
“The egg hatched...” “...the egg hatched... and a hundred baby spiders came out...” (blade runner)
“Who are you?” “A friend. I'm here to prevent you from making a mistake.” “You have no idea what I'm doing here, friend.” “In specific terms, no, but I swore an oath to protect the world...” (continuum)
“It's a goal you won't understand until later. Your job is to make sure he doesn't achieve the goal.” (bsg)