Google does not allow you to send mail on behalf of another user [aka "spoof"] unless you've explicitly been allowed. If you have not been allowed it will rewrite the address to the address of the sending account.
To add an account log into gmail, and go to Settings > Accounts > Send Mail As... when you add an address here gmail will send a message to that address asking for confirmation to allow you to send mail on their behalf.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…