I have had two occasions of getting Error 3204 & stopping my emails. 

However, I have found a solution.  That is, stop using the Hotmail program & use Windows 10 mail program.   Add the problem address in Win 10 Mail program & it will read all messages in problem address & work fine.    It found messages sent after the problem address had stopped working.  Then go back to your Hotmail program & it also has started working.