smbclient + Win 2003 Server = problem!

[Igor]

When trying to mount an SMB share from the server running Windows 2003 
Server, the following messages appear:

otdel4 at nt2 otdel4]$ smbmount //server5/Lotus /mnt/smb/ -o username=otdel4
cli_negprot: SMB signing is mandatory and we have disabled it.
3823: protocol negotiation failed
SMB connection failed

Mounting shares from the servers running other OSes (Windows NT 4.0, 
Windows 2000 Prof., Win 9X/Me) works as expected.
Would you help me? What's wrong?