I am getting this message when emails are sent from my shopping cart. My host is Hostgator. I have not changed anything. Everything was working fine. I have called both my hosting company and my ISP and they both say nothing has been changed on their end. So what could it be?

Thanks for any help.... I have tried everything. When I change it to PHP, I no longer get the error message and it acts like it went through, but the email is never received.

Thank you,
Sharon