Graham, the program used by PHP to send mail is specified in /etc/php.ini, in the sendmail_path setting. You might check there to see what it's set to, and verify that that program still exists in the same location. If it's incorrect, you can edit that file using:There's something undocumented in the High Sierra "security" update too. After installing it on a macOS 10.13 Mac running macOS Server, the built-in PHP can no longer send emails via the local host mail server.
sudo vi /etc/php.ini