I think you have confused the message, not difficult it the heat of the moment with avast's alarm going off.
It sounds like (excuse the pun), the Heuristic checks of the email scanner may consider this suspicious, not that it contains a potential virus. You can check the Heuristic settings and adjust them as necessary to avoid this warning if you regularly send emails like this.
You shouldn't have to disable all scanning of mail, only the outbound scan or adjust the Heuristic settings.