Ok, this is a really quick but (I hope you agree) surprisingly valuable tip. Some email clients and webmail services leak your real public / external IP address by attaching it to outgoing emails. There is now an online tool available to detect whether this issue affects you.
It should be noted that the tool is courtesy of London Trust Media, the company who owns VPN provider Private Internet Access. We did however perform a number of tests when connected to different VPN providers (plus when not connected to a VPN at all). Clearly our email accounts do not leak, and this tool seemed to show no bias towards PIA (other than displaying an advertising banner for the service).
1. Go to http://emailipleak.com.
2. Send an email to the address provided (leaving the Email Leak Test page open).
3. Return to the Email Leak Test page, and see the result.