As i know, in the outbox sitting messages not sended, and the sentbox contains the sended messages. The new writed message go to outbox only if You work without connect, but maybe depend from settings.
When i click on send/receive, from outbox send all messages.(MS outlook express)
Outbox: waiting to send, ready to send, messages not sent due to connection problems ?
Sentbox: a kind of ARCHIVE of messages successful sent.
So if you want to add comment to a message already sent,
or just want resent, the message be found there.