Current mails in queue:
Rest of the funny commands you can find @
Log of processed mails: /var/log/exim/mainlog.
Log of rejected mails:
Spamassassin, pop3 & imap log (old style):
/var/log/maillog (red hat)
If you want to track on of these logs in real time, use this command:
# tail -f
You can quit it by entering control-c or control-z.