set myReply to "Thanks, I've received your email. If I have any questions about it I will contact you directly"

tell application "Mail"
    set theSelection to selection
    if theSelection is {} then return
    activate
    repeat with thisMessage in theSelection
        set theOutgoingMessage to reply thisMessage with opening window
        repeat until exists (window 1 whose name = "Re: " & subject of thisMessage)
        end repeat
        delay 0.1

        -- send gui scripting to handler to get it out of Mail tell block
        my guiScriptMail(myReply)
        send theOutgoingMessage -- This automatically sends the message!
    end repeat
end tell

on guiScriptMail(myReply)
    tell application "System Events"
        tell process "Mail"
            set frontmost to true
            -- keystroke "a" using {command down} -- ⌘A to select all
            keystroke myReply
        end tell
    end tell
end guiScriptMail