We’ve had a receive connector setup forever to allow anonymous relays, but until now have not had a need to allow those relays outside of our own company. When we were trying to send outside of the company were were getting an error in our exchange receive connector logs that read as follows:
550 5.7.1 Unable to relay
Telnetting to the Exchange Server from a relaying server and trying to send to a gmail address made it apparent that exchange did not want to send to an external email address if it could not authenticate the user.
To fix this issue, you can use the following Powershell command:
Get-ReceiveConnector "mailserver01receiveconnectorname" | Add-ADPermission -User "NT Authority\Anonymous Logon" -ExtendedRights "Ms-Exch-SMTP-Accept-Any-Recipient"