If Campground Master suddenly stops being able to send confirmation E-mails and you use a GMail account (using the GMail SMTP server), it's most likely due to the new feature of Google that blocks SMTP for a lot of people until you change a security option.
To solve it:
1. Go to https://myaccount.google.com/
2. Sign in if needed.
3. Click on 'Signing in'
4. Toward the bottom you will find a box/section 'Allow less secure apps'. Make sure it's "on" (click to the right end of the slider, it should turn blue).
Now E-mail should work.
Of course if you ever change your GMail password, then you also need to make sure Campground Master has your new password in Maintenance / Park Setup / SMTP Setup.