Hi Mark,
I am happy to help you today. The first step is to check if there is a Storage issue. You can verify the storage of your Microsoft account from https://onedrive.live.com/?v=managestorage&sw=bypassConfig page.
Second, could you try sending an Email to the account, prefrably from another Email account you have and see if you get any bounceback message.
I’m eager to hear back and work together to find a solution.