If you have multiple email addresses but would prefer to use just one, you can still collect email from your old email addresses by enabling an auto-forwarding feature. This will allow you to still monitor messages coming through your old email address without having to worry about checking different addresses.
Log in to your old email address. If you don't remember your password, look around for a password retrieval link. You will probably be asked a personal question like your mother's maiden name or your favorite pet, and then you will be given your password and will be able to log in.
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Go to the POP/Forwarding area of your email. This is generally in the settings area. Make sure all POP and forwarding settings are enabled. Then log out.
Log in to your new email address and go to the POP/Forwarding area (again, most likely found in the settings area).
Find the receive mail option. You will need to enter your full email address and give it a name. Once you confirm the email address is yours, an email will be sent to the old email address.
Log back in to your old account and follow the verification link. You will need to enter some information about your email client (including SMTP and IMAP information) as well as your password. However, once you have submitted the information, you should start receiving emails from your old address at your new address momentarily.
These steps apply to Web-based email. If you use an email downloading client like Outlook or Thunderbird, you can get mail from an old email address by simply adding it to your list of mail clients (the same way you added your current address.) No special steps are needed.
To retrieve old emails from an address, you will need to manually forward them to your new address by logging in to your old account and then forwarding them to your new account. Unfortunately there is no way to automate this process.