If you send mail via the 'test mail' interface, and there's a problem with the SMTP server, you'll get a warning / error message saying so. If a user does the same thing through the public interface, they are not notified that the mail failed to send.
If you send mail via the 'test mail' interface, and there's a problem with the SMTP server, you'll get a warning / error message saying so. If a user does the same thing through the public interface, they are not notified that the mail failed to send.