I am afraid there is nothing like that when you want to setup notifications.
If you use Mailplus, then you have configure the Mailplus so that atleast it can send emails. The same way I did it for postfix. Just installed Postfix and I had to make sure that the mail command works. If that works, then no extra configuration needed on Checkmk side.
You have to first make this work:
echo “content” | mail -s test-subject xyz@gmail.com