I had this question after viewing exchange external email repeatedly blocked/undelivered
i have a similar issue. my exchange was working good fro past 3 years , but now from past few weeks we are getting rejection messages
Please turn on SMTP Authentication in your mail client. exch2013.kbrokerage.com [126.96.36.199]:35454 is not permitted to relay through this server without authentication.
A problem occurred while delivering this message to this email address. Try sending this message again. If the problem continues, please contact your helpdesk.
i am unable to find out the reason for this. I tried using a new send connector with smart host and basic authentication. before i was using send connector with MX