in order to be work in Todoist, I needed to fully url encode the string (Todoist choked at the @ symbol):

```
tell application "Mail"
	set theSelection to selection
	set theMessage to first item of theSelection
	set theUrl to "<" & message id of theMessage & ">"
	set theUrl to do shell script "/usr/bin/python -c 'import sys, urllib; print urllib.quote(sys.argv[1])' " & quoted form of theUrl
	set theUrl to "message://" & theUrl
	set the clipboard to theUrl
end tell
```

 - I saved the script to `/Library/Scripts/Mail Scripts` as suggested above
 - I installed also FastScript and bound the AppleScript to ⌘⌥L (for **L**ink)

now I can hit cmd-l on any message in Apple Mail and paste the link in todoist and clicking it opens the mail again, thanks internet!