"Outbox"?

if you send a private message and then accidentaly delete it from the "Outbox", does it mean the user hasn't read it at all? Whats the difference between "Outbox" and "Sentbox"?

Messages found in outbox are not jet read and if you send a message and before a user reads it, delete it from your outbox, he wont get it. Messages pending to be read.
Sentbox messages are all messages that have reached their destionation.