See the image here:
http://imagebin.org/83543
On each individual mailing's first configuration page there is a checkbox which says "Show in Archive".
In 6.01 although the checkbox itself can be checked or unchecked, once the mailing has been sent, there is no "save" button so the status cannot be changed from the condition it was in
before sending.
I am sure in 5.5.1. I used to enable "Show in Archive" only after sending because mailings were showing in our public archive list (
http://www.theopenlearningcentre.com/resources/newsletter-archive) even before they had been sent.
Does that make more sense?
Al