this is a forwarded bug http://bugs.debian.org/400734 from a Debian user:
"man 8 mount.cifs suggests that a credentials file can only contain
username and password fields.
It can, however, actually also include a workgroup field.
Which is important if a Windows file server suddenly stops accepting
user@domain as username but insists on username user and workgroup
*** This bug has been marked as a duplicate of 4541 ***