Error when connecting Google's SMTP in Chargebee.
Due to Google's security settings apps that don't use updated security practices like OAuth2.0 over IMAP, in this case, you are using your Gmail SMTP Username & Password.
We were able to find a related forum article, Click Here.
However, turning off the setting for "Less secure app" is not recommended by Google and Chargebee, you can consider using a 3rd party SMTP provider.
If you need further clarification you can write to our support team.