Could it be the Microso$t Exchange bug that was triggered on 01 Jan 2022?
On Thu, Jan 6, 2022 at 4:28 AM MB Software Solutions, LLC mbsoftwaresolutions@mbsoftwaresolutions.com wrote:
Anybody else experiencing errors like this trying to email folks who use Office 365 as their email provider?
Access denied, banned sender[172.104.208.100]. To request removal from this list please forward this message todelist@messaging.microsoft.com. For more information please go to http://go.microsoft.com/fwlink/?LinkId=526653. AS(1410) [BN1NAM02FT015.eop-nam02.prod.protection.outlook.com] (in reply to RCPT TO command)