We had a simple Exchange 2003 setup. One Outlook Web Access Server (Frontend) and on Exchange 2003 server (backend). Our backend server was 5 years old and needed to be replaced. I setup a new server in the domain and setup exchange.
I migrated all of the mailboxes to the new server.
I replicated all of the public folders (adding the new server to the replication tab).
I changed the RUS to point to the new server.
The members of my routing group have my new server as Master now.
We have a Barracuda spam firewall that gets all of our email from the internet. That is now configured to send smtp to the new server.
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