Contact form problem

My contact form seems to work fine (it passes the form data on by email to a number of recipients) but when the form is submitted it returns a message of "Could not instantiate mail function." Any idea what this message refers to or how I can fix it? Thanks

I have the same problem when I’m developing a site, using a temporary IP-based address (1234.1234.1234.1234/~mysite). The email is sent, but I get that message. Once the site is live with its real domain address, the message goes away. I’ve never figured out why it does this.