You're using a relayhost.
It should go out as you@gmail.com because gmail swaps the sender to reflect your gmail account.
You can send email as you@domain.com only if you have a STATIC ip. Dynamic IP addresses are blacklisted by most email providers.
If you have a static IP, just remove: relayhost, *smtp_sasl_password_maps* and generally sasl unless you know what you're doing.
To check wether you're blacklisted or not click here.