Gmail allows external users to send email from its SMTP server.
Sign in to gmail account, then click on the gear on the top right corner, click on "See all settings".
Click on "Accounts and Import" tab on the top.
Click on "Add another email address", in the popup window enter your name and email address, then click on Next.
Enter the SMTP server and username password, then click on "Add Account" button.
First login to google workspace admin panel.
Click on Apps, select Google Workspace, click on Gmail button.
Scroll down the page, and click on "End User Access", then click on "Allow per-user outbound gateways", make sure the checkbox is ticked, then click on Save button.