CRM 2013 Quick Campaign shows completed but only part of emails being sent out

Hello,

My environment is CRM 2013 on premise. I'm trying to run a quick campaign under marketing list. There're 13315 members under marketing list. When I run quick campaign, it took over 3 hours to send 4494 successfully and 18 failures. there're still lots not being sent out. but it shows 'Completed' in Status Reason field. It's wired. Do you know why? is there any limitation for members? Here is the screenshot:

Thanks in advance.