A story posted on hacker news about Google logging every single email you ever wrote is an interesting discussion piece. Of course, that is assuming that someone of the communication chain is using Google's services.
At each point, an opportunity of storing the data is presented. Let's look at the chain. But let me premise that I am not network engineer, so they will be gaps in my analysis.
- Joe sent the email on his computer to his brother Peter. A copy is saved on his email client. Typically in his sent folder.
- The mail server received his email to dispatch to the recipient. The mail server could save a copy of the email.
- The receiving mail server accepts the email and stores it until Peter is ready to take it. The mail server here stores it.
- Peter connect to the mail server and download the email. It is stored in the Peter's inbox.
Just with my simple communication chain, each link provides an opportunity to stored the email.