Rate limiting happens when a recipient mail server doesn't allow emails to be delivered at the same rate as they were sent.
There are limitations to delivery rates imposed by recipient mail servers. Exceeding these limitations results in a practice referred to as throttling. Throttling in terms of email means that a recipient mail server has accepted all the mail it is willing to accept from your IP for a certain period of time.
Please visit our pricing page to see the account specific sending limits.