As a test, I created a new form. I used purely default settings, except under the “Mail” tab, in the “To” box I put in my personal email address. It worked fine. Then I tried two more personal email addresses, and they also worked fine. I then tried multiple emails addresses for my client (several different URLs). None of them worked, so the issue seems to be with the email hosting of my client (NOT Bluehost…). I can email their IT guy, but I have no clue how to explain this to him, other than “your email hosting is blocking form-generated email produced by WordPress.” It’s hard to ask for help when I don’t know how to explain it (and don’t want him getting into the WordPress site and potentially messing things up).