This has everything up and running again, thank you. I did the following:
sudo cp box.mydomain.email-20181106-67f6c4b1.pem ssl_certificate.pem
I’m guessing ‘ssl_certificate.pem’ is supposed to symlink to whatever the most recent Let’s Encrypt cert for the box. domain is. I’ve gotten rid of the symlink for the moment, and copied my primary .pem file over as shown above.