First, you need to tell Gmail how to retrieve your mail from old accounts using POP. Go to Settings —> Accounts —> Get mail from other accounts —> Add another mail account. A new window will pop-up asking you for the email address you're going to import mail from. Fill this in and click on "Next Step."
I found it useful to label incoming messages with “Imported” (use any thoughtful label you can think of), so I could easily identify imported mail later on.
Then, validate with "Add Account" and finally, select “Yes, I want to be able to send mail as email@example.com.” I'm a big fan of using the "from:me" search in Gmail that lets me look for email I sent using any name or address by simply searching “from:me.” I wanted to make sure I could continue to find everything I’ve sent under all my old addresses and aliases, and this last step makes that work.
Sending mail from your old address: If you would still like to have the option of sending mail from Gmail that appears to recipients as if it were sent from your old email address, you can set up a custom “From:“ by going to Settings —> Accounts —> Send mail as:— > Add another email address.
Click on “Next Step” and then “Send Verification.” You'll receive an email with instructions on how to validate your old address. Once it's set up, you can select this address in the “From” field when replying or forwarding.
It might take a while for Gmail to fetch everything from your old account, but the end result was definitely worth it for me.
Source:→ Gmail Blog
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