As reported by kramarz in pull request jenkinsci/email-ext-plugin#109:

The post-send script setting, introduced in JENKINS-30854, disappears when saving job configuration.

A workaround is to directly edit the config.xml job configuration file and include the following line after the <presendScript/> line:

      <postsendScript>$DEFAULT_POSTSEND_SCRIPT</postsendScript>

Afterwards select "Reload Configuration from Disk" from "Manage Jenkins".