Explaining the attribute msexchOrigMDB
In Exchange 2007 and 2003, we have RSG (Recovery Storage group) to recover a database for restoring/merging mailboxes or email messages.
“msexchOrigMDB” is the attribute which tells an RSG database “which database to recover”. It will be the “distinguished name” of the mailbox store which we are attempting to restore by default.
For example – If we have a Storage group named “Third Storage group” and a database in it named “Database_Execs”; when we create an RSG, we have an option to select which database are we planning to recover and this attribute will become the “dn” of that database.
More info found here