Automated submission prevention: case 1 failed

I have a contact form on my website that uses PHPMailer.php to send an email. I am not using SMTP. The contact form normally works but a few times I have got the above error message. It does not seem to be coming from phpmailer (although I could be wrong). Does anyone know what this message means and where it comes from?

It could be caused by anti-spam measures.
If the emails all have the same content, I would recommend using the Bcc mode.

Does this error rarely occur? Does the contact form currently work?

Bye!