Send rate limiting is utilized to throttle, or spread out the sending of a mailing. By default our system spreads out mailings per minute. For example, a mailing size of 100K recipients spread out over 6 hours means 278 messages per minute will be sent. If you are on a Commerce license, you have the option to adjust your own send rate over a 24 hour span. You will find this option in "Finalize" tab when creating a new mailing.
Note: Deferrals or unsuccessful initial deliveries will put the mailing in the default retry schedule. In addition, if an ESP adds their own throttling, the mailing could extend the time period set in Delivra.