Can I move a mailbox while its owner is logged on to it? If I'm in the middle of a mailbox move and a user tries to log on to the mailbox, what's going to happen?

Letting users log on while you're moving mailboxes is a bad idea because that practice greatly increases the risk of corrupting mail data during the move. However, Exchange doesn't do anything to prevent you from moving mailboxes when users are logged on. Depending on when a user tries to log on, the logon attempt might fail—but it might not. Your best bet is to pick a period when you expect few users to log on, then (in advance) send a message to the users whose mailboxes you're going to move. The message should say something like "Between 8:00 p.m. and 2:00 a.m. next Wednesday, you won't be able to use your email. Don't panic." When you're ready to move the mailboxes, add the Logon Only As value to the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\ParametersSystem registry subkey to restrict who can log on to the Information Store (IS). Move the mailboxes, then remove the Logon Only As value. The Microsoft article "XADM: Limiting Logons to the Information Store" (http://support.microsoft.com/support/kb/articles/q146/7/64.asp) explains the process. As always, edit the registry with care because editing the registry incorrectly can cause serious problems that might require you to reinstall your OS.