Each Salesforce Org has its' own rolling 24 hour limit for how many REST API calls can be sent. If the limit is reached, no more API calls are accepted. 

• When sending thousands of emails it's important to estimate the number of REST API calls that will be sent into the Salesforce org. There's a potential to use many API calls for each email (though unlikely as there will be some emails never opened or bounced)  so there is a potential to have an exponential amount of API calls compared to the actual number of emails sent out.
• The daily limit can be found by navigating to Setup>Company Information in Salesforce. 
• Vuture can pause the API queue if you are approaching this limit to save from losing data. 

 

Best Practices: 

• Setup alerts in the case of API limits being reached. In Salesforce this can be done by going into  Setup, enter API Usage Notifications in the Quick Find box, then select API Usage Notifications.
  From the notifications list, you can edit or delete notifications and see details about them, such as who is getting notified, how often, and at what thresholds. Let your account manager know if there's danger of going over the limit so the Queue can be paused. 
• Send large emails in batches as scheduled sends which will help spread out the API calls over a longer time frame. 
• Document and monitor API use from other Salesforce and OnePlace integrations are using API calls to schedule times for large email sends. For example, the Data Loader also utilizes API calls and under heavy use could also breach the API limit.
• If expecting a large amount of Areas of Interests responses, ask your Account Manager to check that 'Use BATCH request' is enabled in Admin.System.Salesforce