You need to wrap the message in quotes. Not sure how postie will deal with the carriage lines though.
A better way of doing it is to write the message to a file, and send the file. Using "RedirectOutput" is a simple way of doing this.
To get your string in quotes, try:
code:
Shell 'postie.exe -host:192.168.4.129 -to:$email1 -from:@userid@@@wksta -s:SU_Not_Installed_to... -msg:"$string"'
