Mailing list form is marking people blank

Apparently people are overlooking the mailing list checkbox in the form and getting marked blank, and since we then have to pay to have the emails cleaned, it costs us money every time it happens.

We finally bit the bullet and payed the penalty fee for the emails yesterday, but I want to prevent it in the future. The only suggestion Vin65 could give me was to add a note asking people to please check the box so they’re less likely to miss it. Has anyone else had this problem, and how did you address it? Did adding a note work for anyone, or do you just eat the fee?