Hi, At site A we have one Exchange server with CAS/HUB/MBX installed; at site B, we have one Exchange 2007 server with CAS/HUB/MBX installed. in case site A is down or internet problem, can we route messages from site A to site B out? How? Thank you.

Try looking at this: http://www.msexchange.org/tutorials/Moving-Exchange-Server-2007-database-Mailbox-servers.html

Yes, You can achive this.. You must have send connector for internet outbound mail in your site A, whenever you have issue of internet connection just create one send connector or can use existing send connector and change smart host entry which is in Site B (Its depend which method you are using for mail flow to internet).. Configuring Exchange 2007 Send Connectors: http://www.petri.co.il/configuring-exchange-2007-send-connectors.htm http://technet.microsoft.com/en-us/library/aa998662(EXCHG.80).aspx Anil