You are offline, away, or blocking the person you are whispering.
One thing that bugs me more about this game, other than the poor matchmaking is not able to defend yourself from toxic whispers. I'm all open for conversations while in online mode. But when the game only allows you to be the receiving end of negative feedback, it irks me. As bad as random people following you on you friends list. (Which is on, can be on another level of harassment. Players following you can see you online, and your location even if you blocked the person)
Sure you could go offline as a blanket fix. But what if your chatting with other people? I've actually been harassed 8 hours after I've done a pvp session.
Block and move on? That only works on individual levels.
I don't know if there is any real down side limiting whispering to only online players. What is the point of a person whispering you if you can't talk to them back anyway.
Frankly people who whisper while offline/blocking you is a cowards way out of dishing out toxic behavior without repercussions/flak/feedback.