Summarize the following:
You may not need to delete your old email address. In fact, leaving it online with proper forwarding will help ensure that you don't miss any important emails in the months following your switch.  There may be no harm in keeping an old address, especially if it's free. You can always use your old address to sign up for mailing lists and other less important accounts, which can help minimize spam on your new account. Keeping your old email account can be very useful if you need to log into an online account you set up with it that you forgot to transition to your new email. If you delete your old account before making the change, you may be locked out without the ability to manage it. You'll want to make sure that you don't miss any important messages, so leave your old account online for at least six months. Web-based email accounts will not deactivate in this time, so you'll receive all of your forwarded messages without having to log into the old address. Many email services allow you to set up an "out of office" or "vacation" message. Use this to automatically inform senders that you have a new email address. You may not want to do this if you get a lot of spam on your old account, as the spam senders will automatically see your new address. See Use Email Autoresponders for information on setting up an auto-reply. If you want to permanently delete your old email account, you can do so once you're sure everything is being sent to your new address. Remember, it may be better to leave the old account active in case you need to access another account set up with it. Deleting email accounts is permanent, and you won't be able to reactivate it once it's gone.  See Delete a Google or Gmail Account for information on deleting your Gmail account. See Delete Yahoo! Accounts for instructions on deleting your Yahoo Mail account. See Close a Microsoft Account for details on deleting a Hotmail, Live, or Outlook.com mail account.

Summary:
Decide if you want to delete your old address. Leave your address for at least six months, even if you plan to delete it. Set up an auto-responder on your old account. Delete your old address when you're sure you aren't receiving important messages on it anymore.