To make the API as explorable as possible, accounts have Test and Live environment API keys. You're not charged any fees in the Test environment, so we encourage you to use it to try out services, perform quality assurance, and run automated testing.
Requests made with Test API keys always validate request arguments, simulate live environment behavior, and enforce rate limits but never print, mail nor, for verification services, verify addresses. The US verification service triggers behavior with specific argument values, and, if you plan on using that service, we recommend reading US Verification Test Environment.