Hello Thank you for your question and reaching out. I can understand you are having query\issues related to Window server keys. There can be MSDN Volume license keys assigned to your Work or School account , Which should not be available after you left Work/School. --If the reply is helpful, please Upvote and Accept as answer--
How to re-activate a product key after wiping in-home lab
I have to rebuild my in-home lab for studying purposes, and when I was still enrolled in college I was given free product keys for Windows Server 2016, I want to re-use my existing product key, looking on the forums I can call Microsoft for further assistance to make sure the key is accepted. But I didnt see any thread talk about cases how the key was obtained.. if that makes sense. Unfortunately due to changes at the college I use to attend, I have since lost access to the email and I want to make sure that if I did decide to re-install a free version of Windows Server 2016 that Microsoft could allow me to re-use the key