Even in Samba 3 the smbmount utility tries to negotiate SMB signing. This will 
not work, as smbfs does not (no longer?) support this. Smbmount will just hang 
after password entry. There is not even some way (at least none that I would be 
aware of) to keep smbmount from using SMB signing.

Thus, if the server is set to signing = mandatory or signing = auto, smbmount 
will fail. As there is no error message on the client side, it is not always 
obvious immediately, why this fails. Only in the server log with a high log 
level one can see, that the client sends a signature with all zeros.

The solution should be to disable SMB signing in smbmount and maybe provide an 
option if one wants to enable it.