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What are the chances my letter was lost? - letter of confirmation by professor

15 days ago, I sent a letter to a former professor of mine in California. I live in Michigan. Since we are using an outdated list I have seen delivery confirmation on whether the letter arrived. I sent Priority Mail and should arrive within 2-3 business days, but there is still no word on the graph. I have since discovered that the address was correct and it is not on the letters to his official name. However, I have not the letter. I do not look like a psycho talker and continue to send things, but I think, is the first that they will never reach. Taking into account the issues of leadership and his time, what are the chances of arrival? I want something on the current address?