Big Business Mail can send emails directly. For Gmail you need to authorize Less Secure App Access to avoid an AUTHENTICATION error (10113, pictured) in Big Business Mail. Go to your Google Account to Security to Turn On or Learn More.
This Solution provides setup details for Gmail. (7078)
If you are using Big Business Mail to send via Gmail you may get an AUTHENTICATION error (10113, pictured). If so go to your Google Account to Security to enable Less Secure App Access.
Keep in mind that Google can automatically turn this setting OFF or remove this option for security. Also it can take a few minutes after making this change for it to take effect. If the error persists, give it a few minutes and try again.