Over deze add-on
You can check the followings:
(1) whether the words you pre-defined exist in the message and the message does not have any attachments,
(2) whether the words you pre-defined exist in the message,
(3) whether the recipients exist or do not exist in the address books.
Finally it confirm you are ready to send message.
Each checkings are enabled/disabled by Check and Send's option. Don't forget setup the checkings before you use this extension.
If you want to see more and latest screenshots, visit my homepage: http://www.geocities.jp/wknxc134/moz_ext/
Please refer to the following link for supported locales: