Duplicate (6,4) Courier-mail crossword.
The answer is CARBON COPY
In e-mail, a carbon copy (abbreviated “cc,” and sometimes “fcc” for “first carbon copy”) is a copy of a note sent to an addressee other than the main addressee. A blind carbon copy is a copy sent to an addressee that is not visible to the main and carbon copy addressees. For example, you may have a work colleague that acts as a back-up when you’re on vacation or not at work.
You don’t necessarily want the people you correspond with to know that you have a back-up. So, to keep your back-up informed, you always send the back-up a blind carbon copy. The fact that a blind carbon copy was sent is not apparent to the main and carbon copy recipients.