Exchange - Unable to send emails

Issue:

Trying to figure out why this was not delivered, and if it was on our end. We don’t seem to be using a connector, and unless it’s an outgoing spam setting, I don’t know why it would be internal.

NDR message: User is not authorized to relay messages through the Office 365 email servers for domain.com.

Resolution:

We checked and found that the NDR which you are getting because the Office 365 settings for domain.com aren't set up correctly to receive and relay messages from you or your organization. 

Need to check setting under domain.com domain. 

Please ask the admin for domain.com, to try one of the following to fix this issue:

Create a connector for the sending domain - To allow messages from the sender's domain to route messages through your organization's Office 365 email servers, create a connector for this mail flow scenario (in the Office 365 Exchange admin center > mail flow > connectors section) and add the sending domain to it. The sending domain must also be set up as an Accepted Domain in your Office 365 configuration, and the sender's certificate subject name must match the sending domain name.

Add the sending IP address or domain to a connector - Create a connector for this mail flow scenario and add the sending server's IP address to its list of allowed sender IP addresses. The sending server will usually be one of your on-premises servers in a hybrid environment, an email server belonging to a business partner, or a smart host email service that you're routing messages through.

 Please find an article for reference :- https://docs.microsoft.com/en-us/exchange/troubleshoot/connectors/relay-access-denied-smtp