In G Suite:

1. Navigate to Apps -> G Suite -> Settings for Gmail -> Advanced settings
2. Check Allow users to send mail through an external SMTP server when configuring a “from” address hosted outside your email domains.

NOTE: It may take up to an hour for the setting to propagate for all users under your account.

In G Mail:

1. Navigate to Settings and click on the Accounts and Import tab
2. In the Check mail from other accounts section, click Add a mail account
3. In the popup, enter the @yourdomain email address.
4. With Import emails from my other account (POP3) checked, click next.
5. Enter email address for username
6. Enter password
7. Enter pop.gmail.com for POP Server
8. Change the Port to 995
9. Check Always use a secure connection (SSL) when retrieving mail
10. Check Label incoming messages if you want to group the messages from your other account into a different label
11. Click Add account
12. It will ask you if you’d like to also send mail as that account, click Yes.

    NOTE: if it does not prompt, go to Send mail as section and click Add another email address

13. Make sure your settings are as follows:

    SMTP Server: smtp.gmail.com
    Port: 587
    Username: full email address
    Password: enter password
    Secured connection using TLS (recommended) is checked

14. Click Add Account
15. Keep the window open until you receive the confirmation email in the account you are adding.
16. Grab the verification code and paste it.
17. Click Verify.

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navigate to Security -> Advanced security settings -> Less secure apps
Check the radio button for ‘Allow users to manage their access to less secure apps’ and click SAVE at the bottom
Go to https://myaccount.google.com/lesssecureapps and make sure Allow less secure apps: ON

In G Mail:
for @gmail.com

1. Log in as the account you are trying to add (ending in @gmail.com)
2. Navigate to https://myaccount.google.com/lesssecureapps and make sure it is ON
3. Navigate to https://accounts.google.com/DisplayUnlockCaptcha and click Continue
4. Log out
5. Log in with your @yourdomain.com account
6. Navigate to Settings and click on the Accounts and Import tab
7. In the Check mail from other accounts section, click Add a mail account
8. In the popup, enter the @gmail.com email address.
9. With Import emails from my other account (POP3) checked, click next.
10. Enter email address for username
11. Enter password
12. Enter pop.gmail.com for POP Server
13. Change the Port to 995
14. Check Always use a secure connection (SSL) when retrieving mail
15. Check Label incoming messages if you want to group the messages from your other account into a different label
16. Click Add account
17. It will ask you if you’d like to also send mail as that account, click Yes.

    NOTE: if it does not prompt, go to Send mail as section and click Add another email address

18. Make sure your settings are as follows:

    SMTP Server: smtp.gmail.com
    Port: 587
    Username: full email address
    Password: enter password
    Secured connection using TLS (recommended) is checked

19. Click Add Account
20. Keep the window open until you receive the confirmation email in the account you are adding.
21. Grab the verification code and paste it.
22. Click Verify.