Both Exmerge and Migrate aim to automate the process of exporting mailbox data to a personal store and then importing the data into a new mailbox on the target server. Manually exporting and importing data is tedious, and the operation becomes more irksome when you're moving mailboxes that contain more than a few megabytes. You'll appreciate the tools if you ever move a mailbox between sites.
Exmerge is newer than Migrate and is my favorite of the two tools, even though Exmerge requires NT 4.0 Service Pack 3 (SP3) and is available only for Intel servers. Migrate runs on both Intel and Alpha systems. You can use Exmerge to move mailboxes between Alpha servers, as long as you run the utility on an Intel server that's running Exchange Server and as long as the Intel and Alpha servers can communicate. To decide which tool works best for you, test both tools and review the available documentation (which is skimpy). You can find information about the utilities in the exchange\docs\exchtool.hlp folder on the resource kit CD-ROM. The Microsoft article "XADM: How to Use the Mailbox Migration Utility" (http://support.microsoft.com/support/kb/articles/q177/7/76.asp) provides tips for running Migrate.