To find an old email address from an email from an Internet service provider, call the provider, and say that you have forgotten your email account. Other email services, such as Yahoo and Gmail, provide online account recovery processes for users who have forgotten their user names.
When calling an Internet service provider, the company may ask you for your account number, an answer to a security question, a user name, your home address or a telephone number. Yahoo requires you to answer secret questions. Gmail emails you a forgotten user name if you registered an alternate email when creating the account. MSN Hotmail does not provide a way to recover an old email address if you do not remember your user name, states eHow.