Problem sending email when updating orders

Hi,

When I go into Customers -> Orders and update the status of an order (eg. from Pending to Processing), and leave the 'Notify Customer' option set to Email, I'm getting an error.

The error varies depending on what I have the Email Transport Method set to in the Configuration -> Email options, but no matter what I set it to I get some kind of error; "Email Error: Could not instantiate mail function. Please check Admin->Configuration->Email Options->Email Transport" or "Email Error: Could not execute: /usr/sbin/sendmail" or "Email Error: Could not execute: /var/qmail/bin/sendmail"

However the emails are actually being delivered - this store is still in the test stages, and I have my own details entered as the customer.

I've checked through everything I can think of and I just don't know where to go next, so any help would be greatly appreciated!

J.

Which hosting provider are you using? Is this Linux or Windows hosting? What version of PHP are you running Zen Cart on?

Hosting Provider - Blacknight

Linux hosting

php version - 5.2.9

ZenCart version / db patch level - both 1.3.9f