I found an email in spam quarantine in the spam log file. The 8.4 score does not tell me why it was quarantined. How can I get more detail about why it was quarantined?

All the log files for spam email are kept here:
/var/spool/qmailscan/qmail-queue.log.x

Each email will get a score and when the score is greater than 5, it is considered as spam and will be quarantined. This value can be changed in one of the files. The scoring method is done by Spamassassin.

You can also check the spam quarentine list to see the content of the email.