The way email works, sending and receiving are totally unrelated. Anyone with a mail server software can send emails pretending to be from your domain (which is why there's so much spam), but receiving is strictly tied to your domain's MX records.
1. Install Postfix, e.g. apt-get install postfix
2. Edit /etc/postfix/main.cf and disable connections from outside
Code:
inet_interfaces = loopback-only
3. Send emails from your Linode!
4. Point your domain's MX records to Google's mail servers
5. Receive emails at Google!