Security feature if opened outside the network

I am trying to come up with a way to protect a workbook if it is opened by someone outside our network. We have come across a situation where the wrong file was emailed out and we did not want those people having access to our model. We would password protect it, but we are constantly in and out of the model that it would get to be a pain to have to keep putting in a password.

If there is some way to password protect it if it sent through email that would be great, not sure how specific excel can get though.


Open to any and all suggestions. Thanks!