I know you want to use the command line, but use the GUI to add one mail account in Apple mail so you can view the sending mail server status. This completes the setup and saves your sending credentials so
mail can use them to log in and send the mail.
Send one mail using the official mail app, then try again from the command line.
If you still get errors from the command line, use the connection doctor to check the results of the sending mail server status. All green lights indicate success - show the logs if you have red or orange lights on the status.
Here are relevant threads if you want to do things the harder, longer way. The first has great test commands and the second sidesteps the native system and installs new versions of mail/sendmail.
The second link installs a complete sendmail package and avoids using the one that comes with the system, thus avoiding it altogether.