i used have one site have two server windows 2003 standrad Edition both are domain controller, one of them have exchange 2007 sp1 installed with mailbox rule,Hub Transport rule and client access while the other server used as a stanby cont. Replication server.
every thing was working fine until we installed a new windows 2003 Entrp edition domain controller with exchange 2007 sp1 with mailbox rule,Hub Transport rule and client access on another site,
than we notice that we cannot send or received mails between the both sites,
the mails are stuck in the queue with error message "451 5.7.3 Cannot achieve Exchange Server authentication"
while the users in the first site can send and received mails from outside.
note that the server in first site has two network interface, the 1st to communicate with its LAN the 2nd to communicate with second site.