This wont be of any help, BUT just sent a email from Outlook 2019 using work email to my live.com account. By the time I opened the Windows Mail app it was already there. Sent a reply back from the live.com via the mail app, closed it and again by the time I opened outlook the reply was sitting there as well, so everything was working 100%.

Edited: took me longer to write this out than do the actual task