Earlier quoted context omitted.
> I'm asking it because I was developing an online forum application that will publicly display your posting IP and registered email address, and sync posts with other sites. Please don't do this. Apart from my own rational self-interest, I'm saying this as a subscriber to various free-software mailing lists, which occasionally get requests from someone to remove archives of support emails they sent 10 years ago. It'…
The thing is that due to the synchronization, users post could be synchronized to another site which they are not registered to. Thus, a mechanism must be setup to allow users to manage their content on other sites. The mechanism that I currently planed was to use email verification (if (User's email === poster's email): They're the owner of the post), which requires all sync sites knows the email address (as an univ…
(If you're worried about not just servers changing but clients changing, and needing to get the key over, there are protocols for efficiently and securely transferring information between client computers, using a very short password that doesn't need to be kept private after the transaction. magic-wormhole implements this sort of thing.)
This may or may not apply to your site, but, in general I don't want other users of a website to be able to PM/email me unless I specifically authorize them to. Requiring that my email is public does not provide me this control. You can do what HN (which doesn't implement PMs) does and let people have an optional public profile, where they can list a way they want to be contacted by other users if they want.