It's the AVG certify message appended to the email that causes it, you need to switch it off:
Right click on AVG system tray icon and select Launch AVG Control Center. Click on E-mail Scanner (on the right) and then click the Properties button (bottom right). In the new window click the Configure button and then untick the Certify Mail boxes that come under the E-Mail Scanning heading (keep the Check Incoming/Outgoing Mail options ticked though). Click OK.