The mail attachement is a string that contrains the path and filename of the file to attach.
Don't forget that the path and filename will be assessed from the server, So if you put an attachement as "C:\MyFiles\File1.txt" then the directory and file MUST exist on the server running SQLServer.
You will probably find the MAPI error is caused by not finding the file to attach.
Hope this helps,
Chris Dukes