Dear maintainers, I almost forgot to write about /var/mail. If I separate it on debian installer, the owner and permission was not correct. Exim cannot send mails. So, root@buster# ls -ld /var/mail drwxrwxr-x 3 root root ... root@buster# chown root:mail /var/mail root@buster# chmod 2775 /var/mail root@buster# ls -ld /var/mail drwxrwsr-x 3 root mail ... And now, I confirmed Mozilla Thunderbird receives logcheck summay mails.