I have had the following response from my hosting company:
"Please include a -f parameter within the mail function of your script. If this is absent the emails generated may be picked up as spam by our outgoing mail server"
I have been unable to locate either the $email_from or the sendmail_from functions in any of the scripts with the contact form solution pack.
My host gave the following example:
mail($email_to, $email_subject, $email_message, $headers, "-firstname.lastname@example.org");
Does this make sense to anyone?