Thank you for reaching out to Constant Contact API Developer Support and for your patience. My team is here to assist outside software developers with questions about building into Constant Contact's API.
Our standard V2 API rate limit is 10,000 calls per day, and 4 calls per second. Your key was occasionally exceeding the calls per second limit. Ideally, the integration you're using should re-try any requests which fail due to this limit. If you've noticed that these requests are not being re-tried, or are concerned that they may not be, you should reach out to the integration's developer.
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