Instead of locking the word document, you can embed it in a pdf that is secure, or using a compression program like zip7 and encrypt the contents of the zipped file. I do not know if zip7 is available for mac, but it is for windows and is free. It requires the other user to also have zip7 to unprotect the contents.
The catch is you need to agree on a password that can not be captured ex. via phone instead of email. or a different email account.
There are programs for sending secure emails as well. I don't really know much about them, but they exist.