If you place a script called ‚Filter‘ in /boot/home/config/add-ons/MailDaemon and set it as executable, it will be run automatically each time the mail_daemon detects new mail.
This script will pop open an alert window specifying who you received e-mail from and offers two options: To open the e-mail to read, or just „OK“ to get rid of the alert box.

Save the script below as /boot/home/config/add-ons/MailDaemon/Filter and mark it executable with

chmod 755 /boot/home/config/add-ons/MailDaemon/Filter

name=$(catattr "MAIL:from" "$1" | sed 's/.* : *//g')
alert --idea "New mail has arrived from: $name" "OK" "Open mail" > /dev
/null
retval=$?
if [ $retval -eq 1 ]
then /boot/beos/system/Tracker "$1"
fi