There is a default limit for sending of emails on every Pepipost account which is also called as warmup limit or daily limit. 

You don’t want to be mistaken as Spammer by ISPs and risk the chances of emails landing in Spam. Pepipost strongly recommends a warm-up plan (daily limit) to systematically scale your email volumes over a period of time.

This helps build your IP reputation and improves email deliverability. The daily limit keeps on auto increasing, based on how well your daily emails are performing.