Thanks for the hint.
This one-line script works fine in the custom rules field:
However, it seems that neither reseller nor client can add such a custom field.
FEATURE REQUEST: allow user to toggle between "send copy" (which is already available as an option), "redirect" and "do not forward" to the optional e-mail address field.
I also tried to simply reduce the quota AND copy mail to second destination, but in case of quota exceeded the copy is also not being sent :-(