I deployed a Microsoft Exchange Server Recently and found out that the it is unable to receive emails. Got the below NDR when trying to send an email out from a Office 365 user.
Since the issue was related to mail server not been able to receive emails, I checked the Hub Transport Servers “Receive Connectors”. as shown below.
Once selected, under each receive connectors properties, I have enabled “Anonymous Users” option under the “Permissions Groups” tab as shown below.
Make sure you do this on both the connectors. Once this has been enabled, Exchange server could receive emails from the external users.