Notes should be able to warn users when they try to send an email with certain words that could indicate an attachment, but there is no file attached.
(This has already been implemented by the Gnome team with Evolution, and works quite well.)
This works by checking for words and phrases like "attachment" and "attached file" in the text of the mail, and then checks to see if there is actually a file is attached. If there isn't a file, it warns the user that they may have meant to attach a file but did not.
(Come on, we've all done it!)
This feature should (like the spellchecker) not bother with the history text in replies or forwarded messages, to avoid tripping over the text put in place of stripped attachments.