1. yesterday I have 500 subscribers
2. a newsletter sent to all 500 subscribers yesterday
3. today I have 600 subscribers (100 new )
4. I cannot send the newsletter to the new 100 subscribers
how can I send the newsletter finished sending before to new subscribers?
You cannot. A campaign (newsletter) is addressed to the subscribers active when you start to send it. Actually it seems you want an autoresponder and not a newsletter.