As with GMail, the Yahoo E-mail service has a new setting that can prevent E-mailing through their SMTP server. If Campground Master suddenly stops being able to send confirmation E-mails and you use a Yahoo account (using the Yahoo SMTP server), it's most likely due to the new feature that blocks SMTP for a lot of people until you change a security option.
3. Toward the bottom you will find a box/section 'Allow apps that use less secure sign in'. Make sure it's "on" (click to the right end of the slider, it should turn blue).