This is using MS Office 365. My user try got this ndr while he tried to send an email to this firstname.lastname@example.org (fake domain). Please see the messages,
Office 365 received the message that you specified, but couldn't deliver it to the recipient (email@example.com) due to the following error:
Error: 550 5.1.0 550#5.1.0 Address Rejected
A non-delivery report (NDR) message was sent to <the sender>. The NDR might provide more details about why the email message wasn't delivered and how to fix the issue.
How to fix it
As the sender to follow the instructions in the NDR to fix this issue. The NDR might also include specific information for email admins. If the sender is unable to fix the issue, ask them to forward you the NDR and then follow the guidance for email admins.
detail Reason 550 #5.1.0 Address rejected
OutboundPorxyTargetHostName = mx.def.com
What shall we do to be able to send mail to the above recipient?
Thanks in advance