Finder to display the contents of a Mail message

Hello, folks. I’m sure this is nothing for you all, but I don’t know what I’m doing. I want to make a rule in Mail that will cause the Finder to display the contents of an email message when it arrives. So if I get a message from a certain sender, up pops a window with the contents of that message. Here’s what I’ve written. Obviously it doesn’t work

tell application “Mail”
set theMessage to message content
tell application “Finder”
display dialog “theMessage”
end tell
end tell

Thanks, all. Sorry if this is mundane and simple.