To allow users to send mail through a third-party SMTP server when configuring a "from" address outside of Google Workspace email domains, you can do the following:
- Go to the Admin console Home page
- Navigate to Apps > Google Workspace > Gmail > End User Access
- In the Allow per-user outbound gateways section, select Allow users to send mail through an external SMTP server
- Click Save
The SMTP relay service is an advanced setup for businesses using Google Workspace.
The SMTP server address is smtp-relay.gmail.com, and the username is the Gmail email address.
The password is an app-specific password generated from the Google account.
The port is 587 (with TLS) or 465 (with SSL).
Try these settings
So first Go to admin Console Home page
Apps Google Workspace then Gmail then End User Access
In the Allow per-user outbound gateways section, select Allow users to send mail through an external SMTP server
then Click Save
In TSS go to tools Options App Settings
Then go to Email & SMS
SMTP server address is smtp-relay.gmail.com, and the username is the Gmail email address.
The password is an app-specific password generated from the Google account.
The port is 587 (with TLS) or 465 (with SSL).
Fill those in to your TSS then open invoice go to Print choose email invoice and put in your email address and it should send you the invoice