Outlook - Cannot open Item, Operation Failed

[debsw1]

Server running sbs2003. Have a problem with either a outlook client or owa client opening emails in a specific mailbox.

It isn't all emails with the problem in the mailbox just a hand-full. When trying to open from outlook you get the error 'Cannot open Item, Operation Failed' Then in OWA you get a HTTP 501 error, not implemented.

Thing is these emails have been opened before with no problems and now all of a sudden you get these error!!!

Any Ideas ?

Thanks
Debbie