Hi. I was experiencing similar, but slightly different problem as you when
using DeleteAttach (). When I would create an attachment, and then delete
the old ones from a message, the original attachment would stay and the new
one would be gone. The behavior was crazy, and not neccesarily worth
explaining here. I was writing an exchange addin the would modify the
attachments in the OnSubmit () event. This is where my problem was. After
looking an example that kinda did was I was doing (http://www.codeproject.
com/com/outlookaddinzip.asp), I tried placing my code in OnWriteComplete ()
and Whala! It worked. So the moral of the story is: Don't modify the
attachments unless your in OnWriteComplete ().

Sorry if this was too unrelated, but I found this post while trying to solve
my problem, and so I thought maybe others would too.

Have a great day!

-Joel