Both mean reply, but can only be used if the reply is transmitted via code (be it letter, e-mail, flag semaphore, Morse code etc. as opposed to speech (including telephone), gesture etc.). Note that does not refer to the object that contains the reply; it refers to the reply.
E->OK
E->OK
->OK
->Weird, speech isn't via code
**EDIT** :
Re: `transmitted via code`, I think the best practical way of defining it would be "not via speech or gesture". To your example, spy talking in code would be because the transport layer (haha you can tell I'm an Engineer) is still speech, but I think it's border line. If they'd do the same on the phone, that will be still . However, if they transmit encrypted message via phone (like by reading out bunch of numbers etc.) or even in person, that would be now because it's no longer via speech.