During migration of users from Exchagne 2003 to exchagne 2007 some time you may see like after user is migrated to exchange 2007 server and if you see the properties of the user , it will still show as Legacy mailbox. If user is still showing as Legacy Mailbox then chances that OWA may not work for that user. User has to be converted to the regular mailbox. Below is the powershell to set legacy mailbox to user mailbox.
Set-Mailbox -id <Username> -ApplyMandatoryProperties