I would suggest posting on the interwebs is more like sending a letter to the editor.
If you don't want it to be public, you shouldn't publish it.
Email communication, in theory, is private, but a public post? Not so much.
Listen to the point ~ it's not where the words are stated ~ (public or private) nor what the occasion was ~ (email or letter), it's WHO uttered the words.
You can be PROSECUTED for certain words (uttering threats, or intimidation, or fraud, or deception, or lies in court) because those words are your words.
Current copyright law gives you ownership of them ~ and if you weren't "selling them" that doesn't mean some yahoo can just come along and "sell them". This company proposes to just take your words and sell them to somebody else.
They are using an entirely different piece of law to justify publishing your letter.