/usr/sbin/courieresmtpd is in courier-mta 0.78.0-2ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/sbin/../lib/courier/courier/modules/esmtp/courieresmtpd
/usr/sbin/../lib/courier/courier/modules/esmtp/courieresmtpd