cant send email from website

[hoangnam8484]

ok, but the problem resolved complete!
But, My child issues:
file log:
Jan 21 15:57:08 webserver1 postfix/virtual[6306]: 83BD9DB8340: to=<user@mydomain.com.vn>, orig_to=<webmaster@mydomain.com.vn>, relay=virtual, delay=0.03, delays=0.02/0/0/0.01, dsn=5.1.1, status=bounced (unknown user: "user@mydomain.com.vn")

1.I have a server mail, used domain: mydomain.com
2. webserver, i have hosting domain is mydomain.com
when i'm send mail from my website to user@mydomain.com, show error unknown user. i want send to my mailserver, dont send to webserver.
So I have to configure like?

Thanks you?