Aha, then you are not using Outlook command line arguments at all but the
MailTo protocol. That is a big difference.
"attachment" or even "attach" is not a valid argument.
See Microsoft's documentation;
http://msdn2.microsoft.com/en-us/library/Aa767737.aspx
and the official RFC documentation concerned about this;
http://ftp.isi.edu/in-notes/rfc2368.txt
It could have been supported in previous versions as a "protocol extension"
offered by Microsoft but was dropped now that Microsoft is more or less
pressured to comply with RFC documentation.
The proper way to do it for an Outlook command line is;
"C:\Program Files\Microsoft Office\Office11\Outlook.exe" /a "C:\My
Documents\labels.doc"
Since you are doing it from a program and if you are the developer I suggest
you use the Outlook Object Model instead of command line arguments.