Any webmail provider can also read the contents of your email. Even if they don't display ads alongside your email content, be aware that they could data mine it to learn all kinds of things about you. If you receive email receipts of purchases that you make, for example, then your webmail provider knows what you're purchasing. They probably know which companies you have business relationships with, where you travel, who you communicate with -- and keep in mind all the things they probably know about the people you communicate with.

This doesn't necessarily bother me. It's convenient to be reminded of an upcoming flight (that was sent to me as an email from a carrier) when I go to run a search for something related like "flight status". That's the kind of automated intelligence that really can make your life better (in small ways). But be aware that there's a lot going on behind the scenes to achieve it.

Imagine the kind of intelligence capability that would exist if you could tap the fact database of a platform like this with AI-style human questions. "OK Google. So, my friend John. What magazine does he read most often?" "I see that New York Times sent him a receipt for his subscription last month. He seems to get a new copy every week by email, and he clicks on 50% of those emails." "Interesting. OK Google, what's John bought from Amazon and Ebay?" "Based on the receipts and shipment emails, John has recently purchased ..." "Where will John be next weekend?" "John has a flight next Friday at 3 PM". Imagine for a moment how much you could probe into other people's lives with the ability to query information like this, and then realize that your email provider has this information about you. Their internal controls prevent you from accessing this information about other people, but they can access it about everyone.

Email is more likely a treasure trove of demographic and other information about you than it is an ad-serving medium directly. This could be true with the browser as well, and is true in limited cases, but from my understanding browser vendors don't currently data mine everything that you browse in general, but their terms of service do allow this with email. (Various Safe Browsing type features do allow HTTP requests to pass through central servers, and browsers like Chrome also track various things about web traffic like what TLS certificates a site presents, to look for MITM attacks. In principle these browsers could track anything/everything. Whatever they track can be observed by the user, though.)

As far as data retention goes, that's hard to know for sure. The data that's displayed to a user is often quite different than how data is tracked in the system underneath. Clicking "Delete" on an email might just set the "deleted" flag in the system. Before relying on retention I'd recommend checking out the terms of service to see what guarantee the company makes about when they'll expunge your content. Without a guarantee in your terms of service, deleted/expired content may as well just be hidden, and there may be a lot more data in the system than you're being shown. Furthermore, even if the data itself was deleted, that does not mean the machine-learned insights from past data have been deleted along with the original data. For example, if you regularly search for programming content on Google, then Google might learn that you're a programmer, and perhaps this could persist even if your individual web searches are deleted.

Service providers are also restrained by their desire not to be creepy. Google might know something about you from reading your email, but might also realize that you'll freak out if they surface that fact obtrusively in the wrong way -- like if you buy a dildo (which they learn by reading your email), they won't start showing you search advertisements for porn. This is the advertising industry and it's obvious that even people who are very into porn and dildos wouldn't want that (all the time). So what kinds of ads that get run, and what kind of insights and features surface from the data they have, will always be subject to human judgment.

By comparison, if you buy a plane ticket (which they learn by reading your email), and search for "flight status", they'll feel comfortable showing you your upcoming flight. The insights available to them far exceed the insights they plug into user-facing features. There are also subtle features they can take advantage of like accurately estimating your age, gender, background, education, whether you own a house, etc., which can increase the precision of various marketing aimed at you. Facebook of course has a lot of these insights as well, through their social graph, though they miss the insights that email conveys regarding your relationship with other businesses. Facebook has parlayed their demographic information about their customers into a tremendous advertising platform, which has the ability to target ads to people in very specific ways based on information about them, which has been until recently unmatched in other platforms.

See things you can target on with Facebook: https://www.facebook.com/business/help/433385333434831

> Custom Audience: Use email addresses, phone numbers, Facebook user IDs or app user IDs to create and save audiences you'd like to show your ads to. Location, Age, Gender and Language: Choose the basic demographics of the audience you want to reach. Interests: Choose specific interests that are important to your audience. These are determined by what people are connected to on Facebook, such as Pages and apps. Behaviors: Select people based on purchase behaviors or intents, device usage and more. These behaviors are determined by what people are connected to on Facebook, such as Pages and apps.

Google's closest comparison AFAIK is "remarketing lists for search ads": https://support.google.com/adwords/answer/2701222?hl=en

Being able to target ads so precisely can considerably increase their value, compared to a broadcast to everyone. Facebook's ability to support such specific targeting is probably a reason behind Google's attempted pivot into Google+: owning a social graph gives you a lot of direct demographics you can't collect easily another way, which then facilitates precise, high-value ad targeting that largely isn't creepy. Data mining people's email might give you the information, but people might not be comfortable seeing a very obvious connection between what's incidentally in their email and an ad or other experience somewhere else. It's not clear how this information in email is being used today, but given that your receipt of an email can impact the behavior of experiences elsewhere on the platform, it's probable that this content is being data mined in various ways.

I haven't reviewed the privacy policy in full, but see the following section: https://www.google.com/intl/en/policies/privacy/

> There are many different ways you can use our services – to search for and share information, to communicate with other people or to create new content. When you share information with us, for example by creating a Google Account, we can make those services even better – to show you more relevant search results and ads, to help you connect with people or to make sharing with others quicker and easier. (...)

> We use the information we collect from all of our services to provide, maintain, protect and improve them, to develop new ones, and to protect Google and our users. We also use this information to offer you tailored content – like giving you more relevant search results and ads.

That's essentially saying that they can do whatever they want with the data (for the legitimate purpose of providing and improving their services). There is no expectation that your email data stays within Gmail - it might improve their search product or your search experience, and so on.

I am largely not bothered by this because I see the mutually beneficial nature of effective advertising: it will help me discover things I'm actually interested in. I enjoy browsing the Steam store to find new video games regularly, for example, and a big part of it are its recommendations (based on what you play, presumably, and have rated) and its promotions. Similarly, I enjoy reviewing my Netflix and Amazon movie recommendations. The more these systems learn about you, the better job that they do finding you cool stuff. Google is the same way: the search gets better and its ability to anticipate your needs gets better. Effective advertising largely does not want to be and does not intend to be manipulative. It serves a purpose of staking the relationship: the product provider puts skin in the game by placing a bet that you'll be interested in it, by spending money to buy your attention. When a consumer finds something they enjoy and purchases it or consumes it for free with ads, everybody wins. There are manipulative elements to classic advertising, but I think the Internet age of personalized, highly targeted advertising is helping business get beyond that.