To find a lost or forgotten email address on Gmail.com, go to the Gmail username-recovery page, choose the recovery method, provide the name of the account, type the CAPTCHA image and then click Submit. The exact process of recovering an email address varies from one email service provider to another.
When recovering a lost or forgotten Gmail email address, the user must provide the alternative email or phone number registered with the account. After providing the alternative email address, Gmail sends the username associated with the lost or forgotten email account to the alternative email address.
If the user provides a phone number, Gmail sends a verification code to the phone number via voice call or text. Enter the verification code and click Continue to receive a list of usernames associated with the lost or forgotten email account. Choose the username from the provided list, then use it to log in to the Gmail account.
On Yahoo.com, go to the Yahoo login page, click on the I can?t Access My Account link located under the Sign In button, tick the check box next to My Account May Have Been Compromised, and then provide the alternative email address or phone number to receive the account-recovery information.