Before doing this migration you have to install the Exchange 2007 in your Exchange 2003 environment
• In Exchange 2007 open Exchange Management console.
• Here select the recipient configuration.
• Select and right click the Legacy mailbox users to migrate them in Exchange2007.
• Select Move mailbox then specify the Exchange2007 server name.
• Then click Finish.
• After that legacy user (Exchange 2003) is converted in to Mailbox user (Exchange 2007).i.e. his mailbox is moved to Exchange 2007 database.
CO-EXISTENCE WITH EXCHANGE 2003 AND EXCHANGE 2007 MESSANGING ENVIRONMENT
• Create some mailbox enabled users for Exchange 2007.
• Send mails between the Exchange 2003 to Exchange 2007 and visa versa.
• Check the mail flow between servers.
In Exchange 2003 even if the users’ access to the mail through OWA and OMA are disabled , the settings are lost on migrating to Exchange 2007.On migration the users are able to access through OWA and OMA
You are not able to access the Exchange 2007 user by using exchange 2003 Front end server