When creating a user registration form, it's possible to check to see if the email or username is already registered even before the user submits the form. Peo

When creating a user registration form, it's possible to check to see if the email or username is already registered even before the user submits the form. People can mitigate this by using email canaries. Email canaries are email aliases that allow for an address to be tied to a specific entity and thusly thrown-away should that entity break their trust. Not all email providers support creating aliases but people should absolutely research which ones do. My question is, do you think this is a privacy risk? Yes. This is why creating obscure canaries are a good idea. Obscure meaning not putting the entity/company name in the address but having a way to map it to said entity. In Fastmail there is a comment field for each alias. In postfix this could be done in configuration comments. Canaries are also important to mitigate some of the cross-site tracking and mapping of people. Emails are used exactly like cookies in this sense. Businesses track and sell who is using what sites so they can better map who can be sold what products. When people use more obvious canaries such as putting the name of the business in the alias it causes these companies to get upset because they have to exclude it from tracking. Some are even starting to label it as fraud despite not being so. This is why canaries/aliases should be obscure and only the creator should know who they map to.