I wonder if its something like:
"By setting the useDomainNameAsHostName attribute to true, you can specify the virtual host and user name by using domain name syntax; this lets you use a backslash character instead of the pipe (vertical line) character; so the syntax would look like "ftp.contoso.com\username" or "ftp.fabrikam.com\username".
"
http://www.iis.net/configreference/system.ftpserver/serverruntime/hostnamesupport
I just gave bing a shot LOL
edit: That page does seem to reference virtual hosts though so I wonder if that applies in this case?