Learn English – How to use the term “carbon copy” in business emails

In my opinion, it depends entirely upon the intended audience.

For a business email, especially one in a 'formal' office (e.g. a government office, a Fortune 100 company, or any office that has people regularly appearing in suit jackets) it is, I think, entirely appropriate to use "whom."

For a personal email, the expectations are rather lower; it depends on your audience and your relationship with them--regardless of my ability, my SO and I have built a sort of private jargon that bears little resemblence to properly written English; "whom" does not often appear in our private correspondance.

For a business email in a less-than-formal setting (or an informal email between close business associates in a more formal setting) similar guidelines may apply.

As I work in an office more to the formal side of the continuum, I have been known to use 'whom' on occasion; nobody has complained yet.

This comes from military, amateur and CB radio communication

"Do you copy?" or "Copy that!" is likely from when a message had to be written down to be shown to a superior officer

http://en.wikipedia.org/wiki/Voice_procedure

Some words with specialized meanings are used in radio communication throughout the English-speaking world, and in international radio communications, where English is the lingua franca.

Affirmative — Yes
Negative — No
Reading you Five / Loud and clear — I understand what you say 5x5.
Over — I have finished talking and I am listening for your reply. Short for "Over to you."
Out — I have finished talking to you and do not expect a reply.
Clear — I have finished talking to you and will be shutting my radio off.
Roger — Information received/understood.
Copy — Mostly used to acknowledge received information. [May also mean Repeat back to me the information I just gave you. ed.]
Wilco — Will comply (after receiving new directions).
Go ahead or Send your traffic — Send your transmission.
Say again — Please repeat your last message (Repeat is not used as it is a specific command when calling for artillery fire)
Break — Signals a pause during a long transmission to open the channel for other transmissions, especially for allowing any potential emergency traffic to get through.
Break-Break — Signals to all listeners on the frequency, the message to follow is priority.

http://www.dyerlabs.com/communications/procedural_codes.html

Copy probably originally referred to writing or typing a received message, but now has is essentially the same as 'Reading you ...'.