/usr/share/doc/cyrus-imapd/changelog.Debian.gz is in cyrus-imapd 2.5.10-3ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/cyrus-imapd/../cyrus-common/changelog.Debian.gz
/usr/share/doc/cyrus-imapd/../cyrus-common/changelog.Debian.gz