The email API to test email-server settings is unlimited and can be used as a email bomb

**Description:** The email-server settings test function in `https://demo.nextcloud.com/xxx/settings/admin/additional` is unlimited and can be used as a email bomb. And the test email API is `https://demo.nextcloud.com/xxx/settings/admin/mailtest` **Reproduce steps:** 1.Go to `https://demo.nextcloud.com/xxx/settings/personal` ,set your personal address to a email address which you want to attack .see screenshot(1) 2.Then go to `https://demo.nextcloud.com/xxx/settings/admin/additional`,`send test mail` ,then above email address will receive an test email. 3.So I can use chrome console network panel to `replay XHR` continuously,then my email box receive many email.see screenshot(2)