i've read on the forums, certainly for previous versions of skype, that if i were to send an offline contact a chat message, and then later change my mind about having sent it, i could simply delete it from my history tab and it would not be sent. in other words, undelivered IM messages that are deleted from history will NOT be sent.
last last night i sent a friend some messages...she was offline... this morning i logged in, she was still offline, and the messages no longer served a purpose, so i deleted them from my history tab.... however, this evening, she got the messages. she promptly replied, and now having checked my history tab.... this last chat session with her is not documented although i have history enabled... it appears the messages that were not meant to be sent were sent, and because they had already been deleted at my end, the "new" chat session was recognised as a continuation of the one from last night and therefore also not documented.