It seems that Microsoft has plans to block email from older Exchange servers; in the words of Steve Gibson:
Microsoft are going to begin blocking incoming eMail to their Exchange Online cloud instanceshttps://twit.tv/shows/security-now/episodes/916?autostart=false
which includes all of Office 365 and Outlook.com, if that incoming eMail originates from other
private so-called on premises Exchange Servers which, while they may be functioning just fine,
are nevertheless, past their end-of-support life.
This is being couched as a way to prevent email vulnerabilities and protect users, but any vulnerabilities are in the software that Microsoft itself produced and sold.
If you run an on-premise MS Exchange Server that is nearing its support end-of-life, then you need to read this article in the Security Now! 916 show notes, available at https://twit.tv/shows/security-now/episodes/916?autostart=false