I am not completely sure of Attachments in Access.. As I avoid using them.. My general understanding of Attachments in MS Access is that, when you add an attachment, the whole file is uploaded and stored as an Object. This object can be view by using the local applications like MS Word, PDF Reader etc.
That is the reason your Attachment fails.. Simple way to achieve attachments is to have a organised structure (Hyperlinks) to store all files under the Server/Shared Network location, thus allowing the file URL to be used for attachment.. Long way to go around is to Save the attachments
to local drives and then use that location for attaching files on Oulook... But I have not worked with this so I am not sure how this code will work for you..
Sorry I could not be of great help..