If you are like I, then while composing a longer message, you repeatedly use the preview function. Unfortunately, if you decide to send your message shortly after your last click of the preview button, then it is discarded as spam. The whole message is lost, you cannot re-send it after waiting a few seconds, you have to write it all again.
Generally it is useful to not allow "rapid-fire-messages", but including the preview button in that calculation is certainly a bug, which I hope won't bite anyone else now
Ulrich
PS: With "Message" I mean personal message, not forum post. I haven't had this problem with forum posts.