Rate limiting allows ESPs proper time to process and filter spam and ensure that transactional email doesn’t get backed up. Without rate limiting in place, ESPs would be even more overwhelmed than they already are.
The ESPs all have different sending limits on a per hour, per day basis. Once you hit thresholds with the rate limits, send too much spam, or have any number of other issues, the ISP may start returning error messages.
Some ESPs will want you to slow down the sending, stop sending for a period of time, or change your habits (due to bad engagement, bad reputation, etc). We automatically adjust your sending rates according to the feedback from these ESPs to keep you in their good graces.
Generally, these rate limits are on a per IP address basis. Contact our Support Team if you wish to purchase additional dedicated IP addresses for your account.