Spam logging for analysis, and probably more in the future.
The log message includes the list of envelope recipients for the message. That means it also includes BCC-recipients, which might pose privacy problems.
warn spam = nobody
add_header = X-Spam-Status: spam\n\
X-Spam-Score: $spam_score
# ...
log_message = Spam received: score=$spam_score; sender=$sender_address; recipients=$recipients
grep 'Warning: Spam received:' /var/log/exim4/mainlog | sed 's/^.* score=\(.*\); sender=\(.*\); recipients=\(.*\)$/\1\t\2\t\3/'