How to check email from other gmail account (POP3 setting)

You have a gmail account 'A'
and also have another gmail account 'B'
Your main account is A, so you want to check the mails from B also in A. (how to transfer emails between two gmail accounts)

These are normal steps that Google tells us;

First, change your settings in your other account:

1. Open the Gmail account you want to import from.
2. At the top right, click the gear .
3. Select Settings.
4. Select the Forwarding and POP/IMAP tab.
5. In the "POP Download" section, select Enable POP for all mail.
6. In the drop-down menu next to "2. When messages are accessed with POP," choose what to do with messages in this account, such as keep Gmail’s copy in the inbox (recommended).
7. At the bottom of the page, click Save Changes.

Then, change your settings in your new account:

1. Open the Gmail account you want to import to.
2. At the top right, click the gear .
3. Select Settings.
4. Select the Accounts and Import tab.
5. In the "Check mail from other accounts (using POP3)" section, click Add a POP3 mail account you own.
6. Enter the full email address of the other account, then click Next Step.
7. Enter your password. If you use 2-Step Verification, you’ll need to enter a new app password.
8. Now you have a few options to choose from. Here’s our recommended settings:
   • Leave a copy of retrieved messages on the server - Leave unchecked. This setting is controlled in the POP settings of your other account.
   • Always use a secure connection (SSL) when retrieving mail - Keep checked.
   • Label incoming messages - Check the box if you’d like to easily see which emails in your inbox came from this account.
   • Archive incoming messages - Leave unchecked. Only check if you don’t want to see new messages from the other account in your inbox.
9. Click Add Account. If you get an error message while getting set up, click the Show error details link to find out more about what’s not working.
10. Once your account has been added successfully, you'll be asked if you want to be able to send mail as this address. This lets you compose messages in Gmail, but have them appear to be sent from your other email account.

Gmail will check your other account for new emails periodically. You can see when the other account was last checked from the "Accounts and Import" tab.

If you only want to import old emails, you can stop importing mail after your old messages have been imported. See directions below.

But This is Not it !! 

If you find this article, you may have problems with that. It doesn't work. It just says "Server denied POP3 access for the given username and password"

This is how I did,and it worked.

Main reason might be that you didn't have 2-step verification on your account B.

Try 2-step verification and set up the App password on account B (that you want to check). Then, you put that 'app password' when you set up the pop3 settings on A account. That's it.

You can find these settings on Google 'My Account' 

https://myaccount.google.com/u/1/security#signin

This problem bugged me for months, and now I made it !!

I hope this help~