I'm not saying the following does happen. It is rather an example of the kind of thing that could happen when an email provider uses the content of your email to show you targeted ads:
1. You are engaged in an email exchange concerning a sensitive topic.
2. Your email provider targets you with an ad based on information associated with that sensitive topic.
3. You click that ad.
4. The advertiser records your IP address. They know what ad campaign the ad you clicked was part of, and what demographic they targeted it to. So, now they know your IP address is associated with that demographic.
5. The advertiser sells their IP demographic data.
6. Others sell IP demographic data that ties your identity to the IP address.
Now someone who buys the right databases can end up knowing that there is a good chance you (by name/address or email address) is likely in a demographic associated with that sensitive topic.
The bottom line is that the data miners are very very clever. They can extract amazing data out of seemingly innocuous data leaks. Clicking a targeted ad is one such data leak.
1. You are engaged in an email exchange concerning a sensitive topic.
2. Your email provider targets you with an ad based on information associated with that sensitive topic.
3. You click that ad.
4. The advertiser records your IP address. They know what ad campaign the ad you clicked was part of, and what demographic they targeted it to. So, now they know your IP address is associated with that demographic.
5. The advertiser sells their IP demographic data.
6. Others sell IP demographic data that ties your identity to the IP address.
Now someone who buys the right databases can end up knowing that there is a good chance you (by name/address or email address) is likely in a demographic associated with that sensitive topic.
The bottom line is that the data miners are very very clever. They can extract amazing data out of seemingly innocuous data leaks. Clicking a targeted ad is one such data leak.