What's the NTDSNoMatch utility?
In Exchange Server 5.5, mailboxes are objects in the Exchange directory. Each Exchange Server 5.5 mailbox object is associated with a Windows NT account, but several mailboxes can be associated with the same account. For example, an administrative assistant might own a mailbox and several resource mailboxes used for scheduling conference rooms, video projectors, and the like. By contrast, Exchange 2000 Server mailboxes are represented as attributes of user objects, so each mailbox object must be directly associated with a user account. When you use the Active Directory Connector (ADC) to link an Exchange Server 5.5 directory with Active Directory (AD), the ADC won't map multiple mailboxes to one user account, so the ADC defaults to creating disabled accounts and associating the unmatched mailboxes with them. However, you can force the ADC to create new, usable accounts and associate them with the extra mailboxes; to do so, you put the string NTDSNoMatch in the Custom Attribute 10 field of the extra mailboxes.
The NTDSNoMatch tool automatically scans the Exchange Server 5.5 directory to look for user accounts that have multiple mailboxes associated with them. It determines which mailbox is the primary mailbox, then produces a Comma Separated Values (.csv) file that when imported, adds the NTDSNoMatch string to the extra mailboxes. Because you can edit the .csv files before importing them, you retain control over which mailboxes are marked. The Microsoft article "XADM: Documentation for the NTDSNoMatch Utility" (http://support.microsoft.com/support/kb/articles/q274/1/73.asp) describes the tool in greater detail. Although the article says that you can obtain the utility only from Microsoft Product Support Services (PSS), it's available from other Exchange-related sites (e.g., http://www.exchange-mail.org/ntdsnomatch.zip).