Well, I isolated it to SQL Server. If you install SQL Server (I downloaded
SQL Server Express for this test), it conflicts with the add-in, so it has
nothing to do with Visual Studio itself. Any ideas anyone?

Michael

"Michael" wrote in message
...
I have written an Outlook add-in using C#.NET using the Outlook XP PIA's.
The add-in puts a toolbar with buttons on the Explorer and most Inspector
windows, amongst other things.

When this add-in is loaded on a machine with Visual Studio 2005 installed
on it, users experience problems in Outlook that do not appear to be
related to the add-in directly. For instance, if they start a new email
message and click the "To..." button, they get the error "The messaging
interface has returned an unknown error. If the problem persists, restart
Outlook.". When they try to open a recurring calendar appointment, they
get the error "Can't open this item. The operation failed".

If the add-in is nor loaded, Outlook behaves normally. Also, if you
uninstalled the SQL Server 2005 that was installed with Visual Studio
2005, Outlook seems to behave normally.

I would not like to tell my users that they have to uninstall SQL Server
2005, so I am hoping to find an explanation that is more specific than
this and hopefully a better workaround. Any help is appreciated. Thanks
in advance.

Michael