How can I avoid emails being bounced back when I send with gmail ?

I am using few gmail accounts to send bulk emails to @facebook.com emails( it works and goes to main email. I tested that and they go to inbox). Gmail can send 500 emails per day.
The problem is that when I send bulk emails , most of the emails I send bounces back and being refused and sometimes goes to spam. I send to @facebook.com emails.
How can I avoid that? Is there any good way to send without facing this problem and get them inboxed?