Everyone wants your email address. Think twice before sharing it.

One technology gaining traction is an ad framework called Unified ID 2.0, or UID 2.0, which was developed by the Trade Desk, an ad tech company in Ventura, California.

Say, for example, you’re shopping on a sneaker website using UID 2.0 when a prompt appears asking you to share your email address and opt in to receiving relevant advertisements. Once you enter your email, UID 2.0 turns it into a token consisting of a string of numbers and characters. This token travels with your email address when you use it to log into a sports streaming app on your TV that uses UID 2.0. Advertisers can link the two accounts based on the token, and they can target you with sneaker ads on the sports streaming app because they know you visited the sneaker website.

Since your email address is not revealed to the advertiser, UID 2.0 can be seen as a step up for consumers over traditional cookie-based tracking, which allows advertisers to access your detailed browsing history and your personal information.

“Websites and apps are increasingly asking for email authentication, in part because there needs to be a better way for publishers to monetize their content that’s more privacy-centric than cookies.” , Trade Desk Marketing Director Ian Colley said in an email. “The Internet is not free, after all.”

However, in an analysis, Mozilla, the non-profit organization that makes the Firefox web browser, called UID 2.0 “privacy regression” because it enabled the kind of tracking behavior that modern web browsers were. designed to prevent.

There are easier ways for websites and apps to track your web activity through your email address. An email can contain your first and last name, and assuming you’ve been using it for a while, data brokers have already compiled a full profile of your interests based on your browsing activity. A website or app may upload your email address to an ad broker’s database to match your identity to a profile that contains enough information to serve you targeted ads.

Ultimately, if you’re wondering why you keep seeing relevant ads despite the rise of privacy tools that combat digital tracking, it’s largely because you’re still sharing your email address.