Mail services are cautions to a new domains which are sending large numbers of emails on a large audience from the beginning. We are helping you to gain your reputation by sending your first emails in small parties, that’s why it takes more time. We call this procedure “Domain warm up”. After first 5000 emails we believe that domain is already warmed and after that sending emails in a normal order.
Email sending could be also stopped if we see some suspicious activity or face with a large amount of spam complains. You can read more about the blocking reasons and how to avoid it in this article.