> The PM (Private Message) is, as you probably know, a lot like regular email. I find it somewhat cumbersome, believing that email is easier.
I agree regular e-mail is much easier. "Cumbersome" is a good description of the PM function! "Painfully slow" also comes to mind.
> As I understand things, here are the functions of those "boxes."
Your descriptions are clear and describe my understanding of them as well. They correspond to the Inbox, Outbox, Sent Items, and custom folders in a regular mail program. The Savebox is just a big "custom" folder where you can save copies of your messages. Nothing goes there unless you mark and save a message in your Inbox or Sentbox. Even though there's a Save button for marked messages in the Outbox, marking a message and trying to save it from there doesn't appear to do anything. It only works after the message goes to the Sentbox.
> If the scale reached 100%, you cannot receive incoming PM form other forum members. So it is important to keep sufficient space available for incoming PMs.
Yes, I saw the meters and that was never the problem in my case. I sent the reply in question at 9:59 yesterday morning, and it was still sitting in my Outbox at 7:50 last night. So I deleted it and sent another copy of my reply. Again, this one was still sitting in my Outbox late last night. However, I just checked at 8:00 this morning, and that message finally made it to my Sentbox. Once it's in there you can mark and save it, and it'll go to your Savebox which is where you can save messages you have sent and received.
Unlike regular e-mail where you control when a message is sent, it appears messages aren't always sent by the PM system in a timely manner or at regular intervals (unless by "regular intervals" you mean once every 12 hours). I did PM myself a "test" message and received it almost immediately although it remained in my Outbox as well. So who knows?
It's difficult to tell when or if a message is actually sent by this system. I don't know if the person received 0, 1, or 2 copies of my reply, but I suppose time will tell. 
Thanks again.