If I need to ensure there's nothing on an image that wasn't supposed to be there, I just copy the visible pixels and paste it to a new document. No EXIF data, outdated thumbnails or anything.
Nope. Microsoft Office has an option/plugin that lets you flag emails and documents as restricted. You can, for example, send such an email to someone else. That person can open the email and use a screenshot tool on that email. What they will get is a flat gray box where the email content was.
Not all pixels on the screen are the same. And you have no idea if your computer tattled on you and let the sending org know that you tried to screenshot the email.
This thread has nothing to do with preventing your source from finding out
This MS Office trick is a quick and easy way to show someone that you can’t trust Windows when it comes to screenshots. Ignore it at your own risk.
The fax should help with that, except that it'll reveal the location of your source fax. Using fax software/service needs its own security...
maybe snail-mail...
This is getting seriously nontrivial
If you do blow the whistel on the government, well, you better trust your journalist partner a lot. Or use a trustworthy lawyer as a go between.
But hey, one could send the fax back and forth, cut of the fax numbers and physically mail it from a very busy train station! And have a someone put the letter into the letterbox for you, someone you pay out of view of any CCTV cameras!