Heads up: this post is a bit old. Caveat emptor.
I tweeted this recently, and it deserves a little more explanation (and permanence):
The key quote:
We’ve now learned Google will also introduce “Confidential Mode,” which lets Gmail users stop recipients from forwarding certain emails, or restricts the ability to copy, download, or print them.
Here's the thing: if you send someone an email – even if it "expires" after a set amount of time, or gmail somehow prevents them from copying/downloading/printing text and/or images from it, they can still share it. All they have to do is take a screenshot – or a literal picture with, say, their phone – and share that.
Look, I took a picture with my phone. This isn't hard.
Sure, this has some utility. For instance, you could share a password with someone with an "expiring" email, and be sure (not that I'd trust Google to actually delete anything...) it won't be sitting in their email inbox forever, just waiting for a future hacker to come along and find it.
But promoting this as a way to prevent sharing of information is misleading, user-hostile, and bad design. Period, full stop.