Please Add Offline Messaging

There are many times I want to send messages to friends when offline, such as asking them when they'll be on again. It's a small unimportant feature but it already kind of exists - Right now the only way to do this is if they messaged me after we were chatting right before I logged off, then I can still reply in the open chat window until I close it, so the feature is already there it just needs to be accessible to send offline at any time.