If you are deploying HMC4.0 in co-existance with HMC3.5, and not a Greenfield install, then read on.
In co-existance when moving all POP, SMTP, IMAP and OWA urls(inc. Outlook Anyhwere/ RPC over HTTPs access) over to Exchange 2007 Hub Transport and Client Access Servers, be aware there is a bug if you have used OWA segmentation using the msExchMailboxFolderSet attribute.
More on Exchange 2003 OWA segmentation here:
The issue is if you use the value '4294967295' (FFFFFFFF) to set 'All Features' for Premium/Gold/'your full access mailbox type here'.
'FFFFFFFF' is read as -1, and Exchange refuses to allow POP3 and IMAP connectivity on the mailbox. Further the Exchange Management Console also flags the 'msExchangeMailboxFolderSet' as corrupt when selecting a user under 'Recipient Configuration \ Mailbox'.
Setting the value to '131071', as mentioned in the article, fixes the issue. (edit: there was an issue with this kb which has now been fixed, so additional notation on 131071 and 1310071 removed).
The way to fix this is to set the attribute to
Other values that actually do restrict OWA use appear to not be affected.
This will also effect any ordinary enviroment that is in transition from Exchange 2003 to 2007 and is using the msExchMailboxFolderSet attribute in this way.
MVP - Exchange Server