We have a single Exchange 2013 server (let's call it SERVER) and a single e-mail domain ("domain.com"). Recently there has been a spate of spam messages being sent from both valid and invalid addresses @domain.com, to valid addresses @domain.com.
First of all, is this something than can and should be blocked? After all, spammers can still send from any other address that's not @domain.com as long as the recipient address is valid, right?
If it should be blocked, what's the proper way to do it? I have already tried the following but it doesn't seem to make any difference:
Get-ReceiveConnector "Default Frontend SERVER" | Remove-ADPermission -user "NT AUTHORITY\Anonymous Logon" -ExtendedRights "ms-exch-smtp-accept-authoritative-domain-sender"
Thanks in advance!