No Mail Intended

I went to my local Post Office, which is run by Hobbits in Middle Earth.
They had hand-painted signs for "National" & "Intentional" mail.
Intentional mail?!
What's that?, I pondered. For I do a lot of pondering these days.
Much too much for my own good really.
What's Intentional Mail?
And what's Unintentional Mail for that matter?
Now I am familiar with Unintentional Email... You know, the times you type off some pissy message then press send and then go: "Bugger! Should NEVER have sent that!"
But I'm not familiar with Unintentional Mail.
How does one go about sending that?
And how does it present itself in the recipient's letter box?
Does it hang limpy half-way through, going "hmmm don't think you really want this...."