Your MailScanner is running as a Zombie
20632 ? SNs 0:02 MailScanner: starting child
18043 ? ZN 0:01 \_ [MailScanner] <defunct>
18092 ? ZN 0:01 \_ [MailScanner] <defunct>
You could try to stop mailscanner see if the zombies are gone and start it again
then keep track of the child procs if they aren't defuncing again. if so, then you have a configuration error in Mailscanner and you should recheck your settings, check the log files for any errors etc ..
If the zombies won't go, reboot the server, it's the only way to get rid of them.
All your mails are on hold atm, you can requeue them again with
after that, keep an eye on your mail.log file and watch for errors.