Unable to Relay emails to External domains (Exchange SMTP Relay)

When you try to relay emails Internal email works fine however unable to send emails to external domains.

Error:

Solution:

Get-ReceiveConnector “ServerCAS01\App Connector” | Add-ADPermission -User ‘NT AUTHORITY\Anonymous Logon’ -ExtendedRights MS-Exch-SMTP-Accept-Any-Recipient

After the change: