The maintainer has closed this bug as 'wontfix', but if an end-user is looking for a work-around, you can add the following to your /etc/logcheck.conf file
# Additional args to mime-construct for sending email MIMECONSTRUCTARGS="$MINECONSTRUCTARGS --type 'text/plain; charset=UTF-8'"It's an undocumented option, so it may stop working with a future version of logcheck, but it works for now.
--- Wade