Ref T10121. This doesn't work at all at HEAD, and even when it did it was mostly just confusing to installs with unexpected setups where Phabricator is receiving mail at @mycompany.com and this is colliding with real addresses.
It might make sense to restore it some day after the next Conphernece update, but just strip it out for now. Since it doesn't work anyway, I'm pretty confident no one is using it.