relay through gmail

[vuthecuong]

I have 1 dynamic IP address at dyndns.com (xxx.dyndns.com)
and 1 gmail account (userA at gmail.com)
And I successfully setup my postfix so that when
I send mail to outside, my postfix will relay through userA at gmail.com.
When recipients received my mails and open them, the mail address
in "from" of theses mails is userA at gmail.com, not
userB at 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
userB at xxx.dyndns.com, not userA at 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