sending email by Relay through ISP smtp

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sending email by Relay through ISP smtp
\ vuthecuong (13 Jul 2007)
. \ Jorey Bump (13 Jul 2007)
. . \ vuthecuong (14 Jul 2007)
. \ (Wietse Venema) (13 Jul 2007)
. . \ vuthecuong (14 Jul 2007)
. . . \ (Wietse Venema) (14 Jul 2007)

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Subject:	sending email by Relay through ISP smtp
Group:	Postfix-users
From:	vuthecuong
Date:	13 Jul 2007

I'm very new to MTA, I have already setup postfix with dovecot in
Freebsd 6.2.
I have not staticIP, I'm using ADSL, so I registered 1 acount in
dyndns.com and have
www.thecuong.gotdns.com.
My Lan's clients have mail address such as:
cuongvt, hanhtm
Because I have no static IP so I can not reverse DNS lookup.
I can send mail using above addresses to mail servers which are not
applying policy
of rejecting email sending from dynamicIP, not static IP.
But if admin of remote mail server setup that his mail server only
receive mail from MTA
that it's DNS can be reverse lookup, I will cannot send to his ail server.
Therefore, I decided to send email relay to ISP as below:
I added in main.cf:
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options =
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd

Then, in sasl_passwd, I have:
omail.fpt.vn cuongvt:minhhanh1!

omail.fpt.vn: my ISP smtp for sending mail to outside.
cuongvt: (real) user name
minhhanh1! : (real) password.
(You can use it for testing for me)

Finally, I execute thse commands:
postmap sasl_passwd
postfix reload

But my thunderbird still cannot relay mail to omail.fpt.vn using
cuongvt＠thecuong.gotdns.com :(
Could anyone tell me what's wrong.
Pls help me.
I really need your help