If you are using an email address hosted by Google (Gmail, G-Suite, etc) and want to use that email from the email sending configuration in your site manager, you need to change your Google account's security settings like below.
1-) Sign in to your email account and go to this page: https://myaccount.google.com/security
2-) Turn off "Use your phone to sign in" and "2-Step Verification".
3-) Turn on Less Secure App Access
4-) Now navigate to https://accounts.google.com/DisplayUnlockCaptcha and click Continue
5-) After making these changes, you can go to your Email Sending Configuration settings and enter the information like below.
Check the option "The server above requires secure authentication (TLS/SSL)"
Email Address: here you put your email address ex; firstname.lastname@example.org
Username: here you put your email address again. It should match the email address you put above.
Password: here you put the password of your email address
Click the button which says Test the SMTP Server Connection. If everything is correct, it will connect succesfully and you will be able to save changes.