Inserting text into the body of an email via a rule

[masaccio]

I had an Entourage rule that edited the body of email according to rules and I’ve just switched to Mail.app and I’m struggling to migrate it. This rule triggers just fine and the value of newContent is correct, but the body of the message doesn’t get changed:

using terms from application "Mail"
	on perform mail action with messages theMessages for rule theRule
		tell application "Mail"
			repeat with eachMsg in theMessages
				tell eachMsg
					tell content
						set paragraphItems to every paragraph
						set newContent to ""
						repeat with theParagraph in paragraphItems
							if theParagraph contains "mydomain.com/support" then
								set newContent to my findReplace(theParagraph, "mydomain.com/support/task?id=", "otherdomain.com/othertask?id=")
							end if
						end repeat
						set last paragraph to newContent
					end tell
				end tell
			end repeat
		end tell
	end perform mail action with messages
end using terms from

on findReplace(the_string, search_string, replace_string)
	tell (a reference to my text item delimiters)
		set {old_tid, contents} to {contents, search_string}
		set {the_string, contents} to {the_string's text items, replace_string}
		set {the_string, contents} to {the_string as Unicode text, old_tid}
	end tell
	return the_string
end findReplace

I’m beginning to think this isn’t possible (changing the text of an email body). Am I right?

Jon