Here are aa few key steps you need to take

• Set up SMTP Relay service here https://admin.google.com/u/1/ac/apps/gmail/routing
  • Allowed Senders: Only addresses in my domains
    Only accept mail from the specified IP addresses: OFF
    Allowed IP addresses:
    Require SMTP Authentication: ON
    Require TLS encryption: OFF
• Ensure “Allow per-user outbound gateways” setting enabled. In your Google Workspace Admin console, go to Apps > Google Workspace > Gmail Settings > Routing.
  Allow users to send mail through an external SMTP server when configuring a “from” address hosted outside your email domain
• Ensure that 2FA is active on the email account used
  • Once 2FA is on ensure you create an App Password
• The SMTP settings in the plugin should be as follows
  • SMTP.GMAIL.COM
  • TLS
  • 587
  • Authentication Turned on
    • Email & Password entered