Current API limits:
- limit on the number of requests from one API Key or from one IP address: 1200 requests / 60 seconds
- for the checkEmail method: 300 requests / 60 seconds
- for the subscribe method: 300 requests / 60 seconds
- for the createCampaign method: 100 requests / 60 seconds
- for the sendSms method: 150 numbers / call
