After an upgrade from Exchange 2003 to Exchange 2007, you should immediately uninstall the Exchange System Manager (ESM) on any desktop in your organisation. This is because the ESM enriches your Active Directory Users And Computers with some Integrated Exchange Tabs. These tabs, that come with ESM 2003, do not work with Exchange 2007. And if they do work, you should not use them :)
A known problem that could occur is when you mail-enable a user on an Exchange 2007 server by using the Active Directory Users and Computers. When you right-click a user and mail-enable it with “Exchange Tasks”, you actually create a Legacy Mailbox instead of a User Mailbox. A Legacy Mailbox stands for an Exchange 2003 mailbox: it will work on an Exchange 2007 server, but you will miss some features, provided by Exchange 2007.
If you happen to have a LegacyMailbox on an Exchange 2007 server, you can upgrade or convert it to a User Mailbox by using the shell command
Set-Mailbox -id Old-Mailbox -ApplyMandatoryProperties
In this command, “Old-Mailbox” stands for the Alias of the mailbox’ owner, i.e. “peter.petrelli”. After executing the command, the mailbox should be converted from Legacy to user in about 20 seconds.