Okay, here’s another tech problem I’m recording here so I can find the solution if it happens again.
I was having trouble receiving mail send from the phpmailer-based contact form on the YTG website.
The problem was that the yokohama-theatre.com domain didn’t have any MX records set up. This is because they are set up elsewhere, but mail sent from inside my webhost is using the webhost’s records, not the public MX records. By adding them in my host’s C-Panel, I solved the problem.