my problem with samba 3.5

[samirasamani]

hi
my problem with samba is about joining to active directory server.
when i enter this command : "net rpc join -S domainname.com -U adminuser"

first samaba request password of adminuser and when i enter correct password samba request another password for host and when enter password for host i get this error :

Could not connect to serverdomainname.com
The username or password was not correct.
Connection failed: NT_STATUS_LOGON_FAILURE
Unable to join domain domain.

plz help me for this problem