By default, Mac OS X Snow Leopard Server (and later versions likely too) doesn’t send any e-mail alerts when a RAID set degraded. Fortunately, sending such a notification can be implemented using a script, as explained in http://serverfault.com/questions/153956/mac-os-x-server-10-6-apples-software-mirrored-raid-worth-it:
# vi /etc/periodic/daily/150.check-raid
# cat /etc/periodic/daily/150.check-raid
#!/bin/sh
# This script checks for any degraded/offline/failed/whatever software
# RAIDs, and if any are found emails a note to an admin. To use it,
# replace the ADMIN_EMAIL value with your own email address, drop it in
# /etc/periodic/daily, and change the owner to root. This’ll make it
# run its check every morning at 3:15am.
#
# Warning: this script doesn’t check anything other than software RAIDs
# built with the Apple (i.e. Disk Utility) RAID tools. It does not check
# any hardware RAIDs (including Apple’s RAID card), or even any third-party
# software RAIDs. If “diskutil listraid” doesn’t list it, it’s not going
# to be checked.
#ADMIN_EMAIL=”youradmin@somewhere.com”
if diskutil listraid | grep “^Status:” | grep -qv “Online$”; then
diskutil listraid | mail -s ‘RAID problem detected’ “$ADMIN_EMAIL”
fi
# chmod a+x /etc/periodic/daily/150.check-raid
Of course this requires that your server can actually send outgoing e-mail messages which may need some manual configuration, as explained in:
Mac OS X Snow Leopard Server: Configuring outgoing SMTP authentication for postfix
[Edit 20130401: Clarified the phrasing, anonymized data]