How to Remove the Last Exchange 5.5 Server

Update the ADC CAs
Ensure that no CAs are configured to use the server you're about to delete. Redirect the CAs to the SRS server for your site.

Check the DLs' Expansion Servers
Ensure that no DLs have listed the server you're deleting as an expansion server. To obtain a list of expansion servers, you need to export DL information to a Comma Separated Value (CSV) file by clicking Tools, Directory Export and selecting the Distribution list check box, as Figure 8 shows. Expansion servers appear under the Home Server column of the CSV file. Be sure to perform the export for all recipient containers in your site. You can also use Directory Export to generate a list of hidden recipients. If someone sends a message to a DL for which you've deleted the expansion server, Exchange won't deliver the message because it won't be able to find the expansion server. . . .