unable to change my user name into postfix

view full story

http://serverfault.com – I have configured my main.cf into postfix, It is sending mail into my gmail account, It is sending mail from [email protected], Now I want that the mail should be sent from [email protected] or [email protected], I am stucking with this problem for more than one hour, but I am not getting the solution for this, Please help me out. Thanks in advance Edit: I have configured it, but at gmail server, sender address is showing [email protected] and I am getting mail delivery fail report from [email protected] (HowTos)