If you're receiving an error when trying to setup your gmail account as a custom email address, then there're few things you could try.

It is recommanded to use smtp-relay.gmail.com as the server relay for when connecting a GSuite account.
Solution A:
Visit https://accounts.google.com/b/0/DisplayUnlockCaptcha and click "continue".
Go back to the settings page and try to save the custom email again.

Solution B:
Go to https://myaccount.google.com/device-activity and allow the access from the less secure app trying to access from Dublin, Ireland. The activity shows as coming from an 'Unknown Device'.

Solution C:
Log in to gmail as domain administrator.
Visit https://myaccount.google.com/apppasswords and generate an app password for Plutio.
Use the app password in plutio email settings instead of the email password.

Solution D:
Go to https://myaccount.google.com/lesssecureapps and enable "less secure apps".
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