[twotwotwo]

Work is the one place this seems OK: if the company provides the email service, it can hide the forward button or whatever if it thinks that advances its interests.

Still think disclaimers about the limits (in the UI, not just the blog post) have to be be stronger. It could help avoid accidental leakage and communicate your intent to keep the contents confidential, but it's absolutely no use against someone hostile. I feel like the name should be more like "mark as confidential" or something, to clearly get across it's a strong suggestion but has no hard enforcement behind it.