/etc/cron.d/greylistclean is in sa-exim 4.2.1-16.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | # If you have configured spamd to run as a fixed user, change "Debian-exim" below.
# Be smart and don't run this as root, it doesn't need those perms
33 * * * * Debian-exim [ -x /usr/share/sa-exim/greylistclean ] && /usr/share/sa-exim/greylistclean
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