Migration from Exchange 2000 to 2007

I am migrating from Exchange 2000 on Windows 2000 to Exchange 2007 on Windows 2008, I am also using Outlook 2003 & 2007.
I have the following issue, in that in migrated mailboxes, receive no new emails, internally or externally, but can quite happily send.
Any help would be appreciated
Thanks in advance.
Simon.