If the current messaging system is a legacy version of Exchange, I make use of the LDIFDE.exe command-line utility to export the current Exchange configuration to a file by executing the following command from a server class domain-joined machine:
ldifde -f ExchConfig.txt -d "CN=,CN=Microsoft Exchange, CN=Services,CN=Configuration," -p SubTree
The output file ("ExchangeConfig.txt") contains a complete snapshot of every Active Directory object and attribute currently set by the legacy Exchange system. This information can assist in the discovery and audit process, as well as provide insight to items that may need to be cleaned up prior to deploying Exchange 2010.
An example of a cleanup item can be any object that references a deleted object. For example, by performing a search for the text "0ADEL," I found the following Mailbox Database using an invalid Public Folder database that has been deleted:
dn: CN=MB 1-1,CN=StorageGroup1,CN=InformationStore,CN=MailServer02,CN= Servers,CN=St. Louis,CN=Administrative Groups,CN=ExchangeOrg,CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=domain,DC=org
Another example of a useful piece of information to gather is the versions of Exchange currently operating in the organization:
dn: CN=ServerName,CN=Servers,CN=St. Louis,CN=Administrative Groups,CN= ExchangeOrg,CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=domain, DC=org versionNumber: 7638