Thanks Sholzy :). Details coming...

v9 implementation attached. Also in git under the email_domain_limit branch. Will be merged into v10 branch.

(Click to enlarge)

(Click to enlarge)

Screenshots attached (config option, and feature in action).

Is this valid only during join? Or is it in effect also after the join process (i.e: when current members modify their profile or change their email address)?

We just did for joining.

My thinking is that this feature is for security, stopping external people being allowed to get into the site.

Do you think it needs doing for account editing also?

I don't see any need for account editing also.

Once you're validated, there should be no reason to limit which email is used, except maybe if it's a corporate site and they wanted email addresses restricted to the corporate email domain for privacy/security reasons (which I think would be an edge case).

Exactly. I thought it was worthwhile to allow admins to edit it to something freely, but I didn't think it was worthwhile to introduce a privilege to decide who could do that (bloat).