relay through gmail

[vuthecuong]

I have 1 dynamic IP address at dyndns.com (xxx.dyndns.com)
and 1 gmail account (us...
<http://groups.google.com/groups/unlock?msg=bb475e48f8724b3f&_done=/group/list.postfix.users/browse_thread/thread/4c122b90b1e5b59a>@gmail.com)

And I successfully setup my postfix so that when
I send mail to outside, my postfix will relay through us...
<http://groups.google.com/groups/unlock?msg=bb475e48f8724b3f&_done=/group/list.postfix.users/browse_thread/thread/4c122b90b1e5b59a>@gmail.com.

When recipients received my mails and open them, the mail address
in "from" of theses mails is us...
<http://groups.google.com/groups/unlock?msg=bb475e48f8724b3f&_done=/group/list.postfix.users/browse_thread/thread/4c122b90b1e5b59a>@gmail.com,
not
us...
<http://groups.google.com/groups/unlock?msg=bb475e48f8724b3f&_done=/group/list.postfix.users/browse_thread/thread/4c122b90b1e5b59a>@xxx.dyndns.com.

So I want to change so that when recipients received my mails and
open them, the mail address in "from" of theses mails is
us...
<http://groups.google.com/groups/unlock?msg=bb475e48f8724b3f&_done=/group/list.postfix.users/browse_thread/thread/4c122b90b1e5b59a>@xxx.dyndns.com,
not us...
<http://groups.google.com/groups/unlock?msg=bb475e48f8724b3f&_done=/group/list.postfix.users/browse_thread/thread/4c122b90b1e5b59a>@gmail.com.

Is there a way to do that?
Regard,
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Below is my config related to above content:
relayhost = smtp.gmail.com
smtp_use_tls = yes
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_CAfile = /etc/postfix/ssl/ThawtePremiumServerCA.pem