Allow me to use my Carnac like insight ;) and give you an answer.
Your own outbound email is fine. However, if it was a good topic going out, then there is no reason to frame it as an email - just frame it as a post.
We can't know of private agreements between people. Take for example an email you receive from a search engine or directory where you have purchased services. That email is covered under the TOS or Licensing agreement you signed when you purchased/sold listings with that search engine. How can we here know that it is a private communication covered by that agreement?
You're right - we can't know what is and isn't covered under privacy, non-disclosure, or trade secrets agreements. If we do allow a email with trade secrets in it to be posted, we might be liable. Once that genie is out of the bottle, you can't get it back in - the damage is done even if the post is removed at the request of the engine.
The only way I know how to make it work is to have one rule forum wide for every one.
Yes, outbound is always fine. I just felt, that if it is an outbound (your creation) email that they want to post here, then there really isn't any reason to frame the post as that of an email, just frame it as a thread starter.
I think you have it knighty. You can summerize an email in your own words - that's fine - but actual quotes just have to be out.
The ones we as the site have to watch out for, are emails from businesses. Those _can_ contain trade secrets. That is an entirely different situation from copyrighted material. Copyrighted material falls under copyright law and you can remove anything in violation after notice. A trade secret is a different situation, where you can't stuff the genie back in the bottle after it's been let out. That's why we can't allow email you receive to be quoted at all.
Your own work, is your own work to do with what you want - as always. (I think that's why the original poster in this thread removed his post - he knew the situation as soon as he left it. I was already head long into a reply when I see he had second thoughts)