To have all your notifications for iCal sent by sendmail through the shell, first create a backup of, then open iCal.app -> Contents -> Resources -> mailScript.applescript and replace the contents with:
set mysubject to "$1"[The 'do shell' command is split onto two lines; enter it on one when you enter the script]
set mybody to "$2"
set myrecipient to "$3"
do shell script "echo "" & mybody & "" | mail -s ""
& mysubject & "" " & myrecipient
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