Exporting mailbox from Exchange 5.5

you cannot move mailboxes across sites.
you will have to log in as the users, make a (personal folder) .pst file of the users mail. then, log in to the other site, attach to the .pst file, and copy the messages to the exch server.

more detailscan be given on each step if needed.

Exchange doesn't offer an automatic way to move users between sites because of the technical problems of transferring mailbox contents in a reliable way. If John Smith moves from server A in site A to server B in site B, any stored mail on server A that refers to him will have an incorrect address. To correct the address, you must either use Exmerge or manually export the mailbox contents to a Personal Folders (.pst) file, delete the mailbox, and recreate it on the target.

Take a look here for assistance with the exmerge utility:

http://support.microsoft.com/default.aspx?scid=kb;en-us;Q174197

http://support.microsoft.com/default.aspx?scid=kb;EN-US;q265441

Delete any spaces in the URLs.

Hope this helps! Good luck!