Do this by rich clicking on an offline person on your contact list, go to Messenger Plus! Features > Set Comeback Message, a window should come up with some areas which you have to type in. Do this, and as soon as they come online (when you're also online), then they will receive the message .
The message will be sent instantly once they sign in (please note that it does it via. messenger plus! and while they're offline, no windows or anything will appear until they come online ).
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