From the error that you have shared, it seems to be an issue with the authentication details that you have provided in the email account configuration. Could you please verify that by clicking on "Test connectivity". You should be getting a "SUCCESS" status if the authentication has passed, otherwise modify the details according to the failure message shown.
It's worth noting that the test connectivity feature does not actually check the credentials provided in all versions, it's just checking connectivity. You can see if this applies to your version by intentionally entering an incorrect password, saving, then testing the connection.