[exim] Problem with smtp authentication

Hi! All:

I have a problem with smtp authentication.
When I use mail command to send mail, I got the following error msg:
SMTP>> AUTH LOGIN
SMTP<< 334 VXNlcm5hbWU6
SMTP>> ********
SMTP<< 334 UGFzc3dvcmQ6
SMTP>> ************
SMTP<< 235 2.7.0 Authentication successful.
SMTP>> MAIL FROM:<root@Latrell> SIZE=1350 AUTH=root@Latrell
SMTP>> RCPT TO:<latrell@???>
SMTP>> DATA
SMTP<< 454 5.7.3 Client does not have permission to Send As this sender.
SMTP>> QUIT

It seems my authentication to mail server is successful. However, when I use root@Latrell as a sender to send mail,
if failed because it doesn't match username/passwd I provided to the mail server. Using root@Latrell (I user root to login to my machine "Latrell") is not a valid sender, thus it complained I can't use it as this sender.
Does anyone know how to fix the problem? Maybe we have two aspects:
1. pass sender check
2. provide sender the same as username/passwd I provided to mail server.
How can I do if any of the two aspects workable? Can I rewrite MAIL FROM address?
Thanks in advance. Any idea will all be appreciated.
My exim.conf about authentication is as follows:

    domainlist local_domains = @
    domainlist relay_to_domains =
    hostlist   relay_from_hosts = 127.0.0.1
    untrusted_set_sender= *
    qualify_domain= zyxel.com.tw
    unknown_username=Latrell.Wang
    acl_smtp_rcpt = acl_check_rcpt

    never_users = root
    ：
    ：

    remote_smtp:
    driver = smtp
    hosts = my.mail.server
    hosts_require_auth = my.mail.server
    hosts_try_auth = my.mail.server
    hosts_override
    ：
    ：
  begin authenticators
      fixed_login:
        driver = plaintext
        public_name = LOGIN
        client_send = : username : passwd
      fixed_plain:
        driver = plaintext
        public_name = PLAIN
        client_send = ^username^passwd

Thanks,
Latrell