Hi, Here's my environment: Site 1 2 x Hub/CAS Transports 1 x Mailbox Server (Windows Cluster) Now I'm in the process of configuring an Exchange Server in a remote site. This server is running all three roles but I'm having problems sending mails between sites. When I try to send a mail it sits in the queue with a "451 5.7.3 Cannot achieve Exchange Server authentication". With site to site communications do I need to configure separate send/receive connectors for Internal relays? Mark

Fixed it. Was a problem with our Cisco ASA's doing SMTP inspection.

Thanks for your sharing.Rowen TechNet Community Support