Getting NDRs when sending to external recipients:
Your message wasn’t delivered because of security policies. Microsoft Exchange will not try to redeliver this message for you. Please provide the following diagnostic text to your system administrator.
The following organization rejected your message: Postini.
Postini #550 5.7.1 Unable to relay – reinject host refused rcpt to: – psmtp ##
Open the Exchange Management Shell from Start -> Programs -> Microsoft
Exchange Server 2007 (or 2010) -> Exchange Management Shell.
Type the following command:
Get-ReceiveConnector “Default SBS” | Add-ADPermission -User “NT AUTHORITY\ANONYMOUS LOGON” -ExtendedRights “ms-Exch-SMTP-Accept-Any-Recipient”