Newsgroups, how to manage downloaded posts
 

I'm on a steep learning curve this week: new machine, new to
Thunderbird, new to Windows 7, ...

I've established a newsreading account (obviously), but find
some basic operations I was used to in Outlook Express seem
to be out of reach.
1) How do I remove messages I'm through with from my downloads?
Every attempt so far has resulted in my being told that
the messages aren't mine, so I can't get rid of them.
2) How do I achieve the equivalent of Outlook Express'
'catch up', which eliminates from consideration all posts
I haven't downloaded?
Thanks for any responses.