Where can you find out which email addresses are not being used?

When creating an email account, email providers prompt users to enter a desired user name and alert the user if the name is already in use. Popular email services like Gmail and Outlook often suggest different available user names that are variations of the words entered.

Most email services have other requirements when choosing a user name. Outlook requires an email address to begin with a letter, for example, while Gmail email addresses must be no fewer than 6 characters long and no more than 30. In addition, both only allow letters, numbers, underscores, periods and hyphens to be used.