You may not want to send your Email to all of your Contacts at once. For some Campaigns, it may be more effective to send to 200 Contacts initially for example, and to an additional 200 every hour thereafter.
In the Campaign Activities widget, click the Green Add Activity button. In the Step 1. Select the Trigger event dropdown, choose On Campaign Start.
In the Step 2. Select the Action to run dropdown, choose Send Mail to Audience and select the Email you wish to send.
In Step 3. Choose Scheduling Options, tick Throttle Send...You can now choose the amount of Contacts you want to send to every minute/hour/day. You can also choose to send to Contacts between certain designated times, for example between 9:00am and 6:00pm, when your Email is most likely to be read.