You may have multiple Gmail or Google Apps email accounts. Managing multiple such emails is difficult sometimes. You may want to consolidate them in a single Gmail account and still send and receive emails with each of the IDs from the same login.
If you own a domain, you can switch to Google Apps and have email at your own domain provided by Google for free. You can consolidate all your other email accounts into this email account and use Gmail effectively.
For example, I consolidated all my gmail accounts into my ananthapuri.com Google Apps domain. I have imported all my old emails from Gmail and other accounts into the new one.
How to Import All Emails from One Gmail Account to Another?
Login to your old Gmail (or Google Apps) account. This is the email account where you want to forward or import all emails from.
Go to Settings -> Forwarding and POP/IMAP tab
In POP Download section, select Enable POP for all mail (even mail that’s already been downloaded)
Save Changes and Logout.
Now logon to your new Gmail (or Google Apps) account. All emails from your old account will be imported here.
Go to Settigns -> Accounts
Under Get mail from other accounts section (download mail using POP3), click on Add another mail account
In the popup window, enter your old email id and click Next Step
In the Username field, enter your Gmail or Google Apps account, full email address.
Select Always use a secure connection (SSL) when retrieving mail.
Click Label incoming messages. This will mark all imported emails with this label for easy identification.
POP Server: pop.gmail.com
Save the settings and you are ready to download / import all emails from the old account to the new one.
Now onwards you will receive all emails sent to the old email address in the new mailbox. You can reply to the old email as well.