Can't get FormMail.pl to work

Hi
I have used FormMail.pl (from this page https://www.inmotionhosting.com/support/edu/website-design/adding-contact-forms/creating-a-contact-form-with-formmail) for a contact form but I get an error message when I fill in the form on this page: http://blackstanniland.com/landregistrycompliantplans.html.

I am returned the following error message:

Application Error

An error has occurred in the program

close sendmail pipe failed, mailprog=[/usr/lib/sendmail -oi -t] at (eval 8) line 108.

Can you help? I am trying to send the form to at least 2 email addresses,

Thanks
Rachel