I'm in the process of migrating from Exchange 2003 to Exchange 2007. There is currently one exchange 2003 and one exchange 2007 server up and running. I can send email from Exchange 2007 to Exchange 2003 but I cannot do the reverse. The mail gets stuck in the "Messages Queued for deferred delivery" queue. I think this has something to do with the fact that I only have the Default SMTP Virutal Server, on the 2003 server, and it is set to use a smart host. This is so Message Labs can scan our outbound email.