I cannot send email via webmail

["sato x" <gladiol4@gmail.com>]

Hi guys,

I have installed qmail-ldap and it's running well. Then I install
courier-imap version 4 and squierrelmail. I leave the default
installation and I don't use /var/qmail/bin/auth_imap to authenticate -
because it's failed all the time, then I can login to webmail with
username@domain.co.id. I can find new emails there, but when I try to
reply or just send email, the server replied with 

  Authentication failed
  Server replied: 535 authentication failure

Is it because I didn't use /var/qmail/bin/auth_imap, since if I login
via thunderbird I use username instead of username@domain.co.id, but if
I login via webmail I must use a complete email address? Below is
squierrelmail config file

SquirrelMail Configuration : Read: config.php (1.4.0)
---------------------------------------------------------
Server Settings

General
-------
1. 
Domain                
: xxx.xxx.xxx.xxx
2.  Invert Time            : false
3.  Sendmail or SMTP       : SMTP

SMTP Settings
-------------
4.   SMTP Server           : localhost
5.   SMTP Port             : 25
6.   POP before SMTP       : false
7.   SMTP Authentication   : login
8.   Secure SMTP (TLS)     : false
9.   Header encryption key :

Thank you in advance for your help.

Best regards,

sato