To use certain functions in the public website builder (like Newsletter sending) a custom SMTP server will need to be added. It is recommended to use a club Gmail account for that, which many clubs have. Follow these instructions to do so:
Log in to the club Gmail account
In the URL paste this in: https://myaccount.google.com/apppasswords
Generate a new app password
Go into the public website builder
Go to Settings > General Settings > Method for sending emails
Select SMTP and enter the following:
SMTP Hostname: smtp.gmail.com
SMTP Port: 465
SMTP Username: [club Gmail address]
SMTP Password: New app password created in #3 above
SSL Connection: Checked
That should be it. After setting that up try sending a newsletter.