Did you try the Microsoft fix:
http://support.microsoft.com/default.aspx/kb/941090
"Messages may be stuck in the Outbox when you use Windows Mail on a
computer that is running Windows Vista"

Although Outbox is the only folder mentioned, in fact it works with all folders.

Gary VanderMolen


"CIRRUS" <CIRRUS@discussions.microsoft.com> wrote in message news:5F445620-FBDB-4E70-B1F9-55427BBA808A@microsoft.com...
> I have no problem deleting from the Inbox, but have problems deleting
> permanently from the Deleted Items folder. Some of the Deleted Items can be
> deleted permanently, but the list of those that can't is increasing daily.
> Highlighting messages in the Deleted Items folder produces a "Message could
> not be displayed" notification where the content of the message should
> be...........and any attempt to delete it produces "An unknown error has
> occurred".
>
> "Robert" wrote:
>
>> I did find a patch in the Microsoft in one of the microsoft communities
>> section call: Windows6.0-KB941090-v2-x86 and I could erase the message in
>> the inbox that couldn't be erased
>>
>> "CIRRUS" wrote:
>>
>> > Following advice, I downloaded Windows Live Mail (which is still a beta
>> > version) to replace Windows Mail. Perhaps it's because I'm working with vista
>> > that I've still got problems. Windows Live does indeed appear to allow
>> > deletions without any problems arising, but I have found that it won't import
>> > all the message files from Windows Mail. About a quarter of them were
>> > imported correctly before I received a message saying that I'd have to import
>> > the remainder manually. I tried the "manual import" but all that happened was
>> > that a folder with the same name as the one from which I wanted to import was
>> > created in Windows Live.........but the emails were not transferred across to
>> > it. Until these bugs are resolved I'm reverting to Windows Mail as my default
>> > mail service.
>> > "Robert" wrote:
>> >
>> > >
>> > >
>> > > "Gary VanderMolen" wrote:
>> > >
>> > > > In that case, my previous advice still holds. Live with it a few more
>> > > > days, then install Windows Live Mail when Microsoft makes it
>> > > > available for download.
>> > > >
>> > > > Gary VanderMolen
>> > > >
>> > > We're september 19 2007 and I"ve made all my update and I got this problem
>> > > in my inbox this morning.