Took me a little time to work out that when you say "forum email" you're referring to what most of the forums refer to as a "private message", a mechanism that is entirely internal to the forum and does not use email addresses.
The solution to your problem may be to never refer to "Forum Email" and never to a "Private Message". It will only confuse and make people think it is Nabble's version of private messaging within the forum. Avoid saying something like, "I'll write to you privately". Say instead "I'll email you". It shouldn't be too difficult for people to appreciate that email means email!
If your users don't realise it, you do need to point out that Nabble does not offer private messages and when your forum invites someone to send an email - that's just what it means.
As you know, the only special thing Nabble's system does is cloak email addresses so they can't be harvested by spammers, but sending an email to someone is sending an email - with all the normal implications - i.e. the recipient will learn the email address that you registered on the forum.
From what I remember of answers to other users questions about private messaging, I believe Nabble said it was a very long way down the roadmap, if it was on the map at all - but I'm sure Hugo will correct me! :-)
Volunteer Helper - but recommending that users move off the platform! VIst GregHelp to discuss Nabble Alternatives.
We never implemented private messaging because we never really understood why it is needed. We all have emails and checking just one inbox seems to be the simplest solution ever. Another forum inbox would be a duplicate and most users would probably forget to check those messages in a timely manner. About revealing emails, we could change the "Send Email to..." screen and add a line that explains what is going to happen. So users wouldn't send emails if they don't want to expose their personal emails in this process. Does that make sense?
I think it's just a matter of so many other forums (at least ski forums) offer separate private email. I'd say half of my members don't want their identities revealed. Some people have their names in there email address.