Republished by Slate. Translations available in French (Français), Spanish (Español), Chinese (中文)

For almost 15 years, I have run my own email server which I use for all of my non-work correspondence. I do so to keep autonomy, control, and privacy over my email and so that no big company has copies of all of my personal email.

A few years ago, I was surprised to find out that my friend Peter Eckersley — a very privacy conscious person who is Technology Projects Director at the EFF — used Gmail. I asked him why he would willingly give Google copies of all his email. Peter pointed out that if all of your friends use Gmail, Google has your email anyway. Any time I email somebody who uses Gmail — and anytime they email me — Google has that email.

Since our conversation, I have often wondered just how much of my email Google really has. This weekend, I wrote a small program to go through all the email I have kept in my personal inbox since April 2004 (when Gmail was started) to find out.

One challenge with answering the question is that many people, like Peter, use Gmail to read, compose, and send email but they configure Gmail to send email from a non- gmail.com “From” address. To catch these, my program looks through each message’s headers that record which computers handled the message on its way to my server and to pick out messages that have traveled through google.com , gmail.com , or googlemail.com . Although I usually filter them, my personal mailbox contains emails sent through a number of mailing lists. Since these mailing lists often “hide” the true provenance of a message, I exclude all messages that are marked as coming from lists using the (usually invisible) “Precedence” header.

The following graph shows the numbers of emails in my personal inbox each week in red and the subset from Google in blue. Because the number of emails I receive week-to-week tends to vary quite a bit, I’ve included a LOESS “smoother” which shows a moving average over several weeks.

From eyeballing the graph, the answer to seems to be that, although it varies, about a third of the email in my inbox comes from Google!

Keep in mind that this is all of my personal email and includes automatic and computer generated mail from banks and retailers, etc. Although it is true that Google doesn’t have these messages, it suggests that the proportion of my truly “personal” email that comes via Google is probably much higher.

I would also like to know how much of the email I send goes to Google. I can do this by looking at emails in my inbox that I have replied to. This works if I am willing to assume that if I reply to an email sent from Google, it ends up back at Google. In some ways, doing this addresses the problem with the emails from retailers and banks since I am very unlikely to reply to those emails. In this sense, it also reflects a measure of more truly personal email.

I’ve broken down the proportions of emails I received that come from Google in the graph below for all email (top) and for emails I have replied to (bottom). In the graphs, the size of the dots represents the total number of emails counted to make that proportion. Once again, I’ve included the LOESS moving average.

The answer is surprisingly large. Despite the fact that I spend hundreds of dollars a year and hours of work to host my own email server, Google has about half of my personal email! Last year, Google delivered 57% of the emails in my inbox that I replied to. They have delivered more than a third of all the email I’ve replied to every year since 2006 and more than half since 2010. On the upside, there is some indication that the proportion is going down. So far this year, only 51% of the emails I’ve replied to arrived from Google.

The numbers are higher than I imagined and reflect somewhat depressing news. They show how it’s complicated to think about privacy and autonomy for communication between parties. I’m not sure what to do except encourage others to consider, in the wake of the Snowden revelations and everything else, whether you really want Google to have all your email. And half of mine.

If you want to run the analysis on your own, you’re welcome to the Python and R code I used to produce the numbers and graphs.