Before you Start reading, I tell you that the post will be a few long, before quit, read it till the end because it will be very usefull in the future.
The best way for a System administrator to migrate Mailboxs or mail messages, is by using Exchange Merge utility (EXmerge.exe). You will find this tool in your Exchange CD in the Support tools.
But before you can use the Exchange Merge utility, you must ensure that you have the correct permissions.
The first permission you must have is "Full Mailbox Access" in the user mailbox. Of course you don't want to set this permission manually in every mailbox that you want to migrate, and of course Exchange is prepared for that. You are "Exchange Full Administrator", right? So you have "Full Mailbox Access" in every Exchange Server mailbox by default.
Assuming that you are "Exchange Full Administrator", and you didn't have to do anything for the first permission, in the second permission you will have to change one thing.
The Exmerge utility work like this. It will access to the source mailbox, will read the information, put it in a PST file and send all the information from the PST file to the destionation mailbox. But if you try this without changing more anything, you will be unsuccessfull. The Exmerge utility, to work right, also need the "Receive As" and "Send As" permission in the user mailbox.
To configure your Exchange Server to enable the "Receive As" as "Send As" permission for your Administrator Account, check this knowledge base article:
Microsoft Knowledge Base Article - 292509
Also check this article, for you to understand what is Exmerge utility:
Microsoft Knowledge Base Article - 265441
Best regards,
Luís Rato
MCP;MCSA;MCSA Messaging;MCSE;CIWA;CIWP;CIWSA;MCIWA