JoomlaCode entry: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33860

@Kubik-Rubik i can confirm the issue and the fix

I'm thinking that we should just revert the actual check in the controller and fix it there. It's not necessary a given that any contact form uses JForm to create that checkbox.
HTML Checkboxes are a funny thing. The value of the checkbox actually doesn't matter at all. You have to check if they are submitted, not if they contain a value. The value is only relevant if you have multiple checkboxes with the same name.
On a sidenote, that's actually done wrong in 3.x as well...

@Bakual

I think that a value parameter should be used in a checkbox field and so we can use this solution to fix the bug!

Yes, we could also only optimize the if statement. Instead of checking !empty($data['contact_email_copy']) we could just check whether this request variable is set at all isset($data['contact_email_copy']). If the checkbox is selected, then the request variable is set with an empty value. With the current check this can never be true without providing a value. In Joomla! 3.x the value is set to 1, so the initial fix does work there.

Just before I test, should I test the value 1 or the proposed controller change? Confused.

I would use isset personally. Mainly because we don't really know where the form is coming from. It may well be an override (or even a module) which doesn't use JForm and thus would still be broken.
I don't want to risk another broken thing :)
The XML change can of course be kept as well. It's always better :)

@Bakual Correct, updated the PR.
@roland-d Now you can test the whole PR. :-) Thanks!

@test: After applying the patch, I do get a copy of the message sent via the contact form.

JC tracker moved to RTC.

Merged with 09e8d05