Issue
Client is trying to setup an external account to send email as their MC account.
Client is getting error message: "Authentication failed. Please check your username/password and Less Secure Apps access:
Server returned error: "535-5.7.8 Username and Password not accepted. Learn more at 535 5.7.8 https://support.google.com/mail/?p=BadCredentials k11sm2783653qkh.51 - gsmtp code(535) "
Solution
Using the following instructions to setup 2 Factor Authentication and generate an app specific password.
Click here to obtain instructions for Gmail Access to less Secure Apps