When installing Soapbox Mailer from the AppExchange, you might receive a message that says, "Custom Roll-up Summary Field Limit Exceeded". This message is displayed because the addition of the 5 custom roll-up summary fields that comes with Soapbox Mailer puts you over your organization's limit for the Campaign object in Salesforce.
Salesforce provides you 10 custom roll-up summary fields per object to start. If you need additional custom roll-up summary field, you can submit a case to your Salesforce account executive and ask for additional custom roll-up summary fields to be allowed for your Campaign object. Asking for your limit to be increased by 5 (for a total of 15 custom roll-up summary fields on your Campaign object), probably would work just fine.